chore: fix broken links (backport #2511)

.github/PULL_REQUEST_TEMPLATE.md

@@ -20,7 +20,7 @@ write a little note why.
 
 - [ ] Targeted PR against correct branch (see [CONTRIBUTING.md](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#pr-targeting))
 - [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
-- [ ] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md).
+- [ ] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/building-modules/10-structure.md).
 - [ ] Wrote unit and integration [tests](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#testing)
 - [ ] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`)
 - [ ] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).

docs/DOCS_README.md

@@ -96,7 +96,7 @@ We are using [Algolia](https://www.algolia.com) to power full-text search. This
 ## Consistency
 
 Because the build processes are identical (as is the information contained herein), this file should be kept in sync as
-much as possible with its [counterpart in the Cosmos SDK repo](https://github.com/cosmos/cosmos-sdk/tree/master/docs/DOCS_README.md).
+much as possible with its [counterpart in the Cosmos SDK repo](https://github.com/cosmos/cosmos-sdk/blob/main/docs/README.md).
 
 ### Update and Build the RPC docs
 

docs/ibc/apps.md

@@ -462,4 +462,4 @@ callbacks](https://github.com/cosmos/ibc-go/blob/main/modules/apps/transfer/ibc_
 
 ## Next {hide}
 
-Learn about [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/intro.md) {hide}
+Learn about [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/building-modules/01-intro.md) {hide}

docs/ibc/apps/apps.md

@@ -0,0 +1,51 @@
+<!--
+order: 1
+-->
+
+# IBC Applications
+
+Learn how to build custom IBC application modules that enable packets to be sent to and received from other IBC-enabled chains. {synopsis}
+
+This document serves as a guide for developers who want to write their own Inter-blockchain Communication Protocol (IBC) applications for custom use cases.
+
+Due to the modular design of the IBC protocol, IBC application developers do not need to concern themselves with the low-level details of clients, connections, and proof verification. Nevertheless, an overview of these low-level concepts can be found in [the Overview section](../overview.md).
+The document goes into detail on the abstraction layer most relevant for application developers (channels and ports), and describes how to define your own custom packets, `IBCModule` callbacks and more to make an application module IBC ready.
+
+**To have your module interact over IBC you must:**
+
+- implement the `IBCModule` interface, i.e.:
+  - channel (opening) handshake callbacks
+  - channel closing handshake callbacks
+  - packet callbacks
+- bind to a port(s)
+- add keeper methods
+- define your own packet data and acknowledgement structs as well as how to encode/decode them
+- add a route to the IBC router
+
+The following sections provide a more detailed explanation of how to write an IBC application
+module correctly corresponding to the listed steps.
+
+## Pre-requisites Readings
+
+- [IBC Overview](../overview.md)) {prereq}
+- [IBC default integration](../integration.md) {prereq}
+
+## Working example
+
+For a real working example of an IBC application, you can look through the `ibc-transfer` module
+which implements everything discussed in this section.
+
+Here are the useful parts of the module to look at:
+
+[Binding to transfer
+port](https://github.com/cosmos/ibc-go/blob/main/modules/apps/transfer/keeper/genesis.go)
+
+[Sending transfer
+packets](https://github.com/cosmos/ibc-go/blob/main/modules/apps/transfer/keeper/relay.go)
+
+[Implementing IBC
+callbacks](https://github.com/cosmos/ibc-go/blob/main/modules/apps/transfer/ibc_module.go)
+
+## Next {hide}
+
+Learn about [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/building-modules/01-intro.md) {hide}

docs/ibc/integration.md

@@ -139,7 +139,7 @@ func NewApp(...args) *App {
 
 ### Module Managers
 
-In order to use IBC, we need to add the new modules to the module `Manager` and to the `SimulationManager` in case your application supports [simulations](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/simulator.md).
+In order to use IBC, we need to add the new modules to the module `Manager` and to the `SimulationManager` in case your application supports [simulations](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/building-modules/13-simulator.md).
 
 ```go
 // app.go

docs/ibc/overview.md

@@ -61,7 +61,7 @@ must use for committing IBC messages is defined in
 [ICS-24](https://github.com/cosmos/ics/tree/master/spec/core/ics-024-host-requirements) and the proof
 format that all implementations must be able to produce and verify is defined in this [ICS-23 implementation](https://github.com/confio/ics23).
 
-### [Capabilities](https://github.com/cosmos/cosmos-sdk/blob/master/docs/core/ocap.md)
+### [Capabilities](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/core/10-ocap.md)
 
 IBC is intended to work in execution environements where modules do not necessarily trust each
 other. Thus IBC must authenticate module actions on ports and channels so that only modules with the

docs/ibc/relayer.md

@@ -7,7 +7,7 @@ order: 4
 ## Pre-requisites Readings
 
 - [IBC Overview](./overview.md) {prereq}
-- [Events](https://github.com/cosmos/cosmos-sdk/blob/master/docs/core/events.md) {prereq}
+- [Events](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/core/08-events.md) {prereq}
 
 ## Events
 

modules/apps/transfer/spec/06_metrics.md

@@ -4,7 +4,7 @@ order: 6
 
 # Metrics
 
-The transfer IBC application module exposes the following set of [metrics](https://github.com/cosmos/cosmos-sdk/blob/master/docs/core/telemetry.md).
+The IBC transfer application module exposes the following set of [metrics](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/core/09-telemetry.md).
 
 | Metric                          | Description                                                                               | Unit            | Type    |
 |:--------------------------------|:------------------------------------------------------------------------------------------|:----------------|:--------|