docs(x/photon): add module documentation (#74)

As per the title, add the `x/photon` module documentation with
references to the photon ADR

x/photon/README.md

@@ -0,0 +1,91 @@
+---
+sidebar_position: 1
+---
+
+# `x/photon`
+
+## Abstract
+
+This module manages the PHOTON token (base denom `uphoton`), introduced by
+[ADR 002](../../docs/architecture/adr-002-photon.md) as the only fee token in
+AtomOne. PHOTONs are minted by burning ATONEs at a rate capped by a maximum
+PHOTON supply of 1 B. Minting is achieved via a single message
+(`MsgMintPhoton`), and the module enforces PHOTON fees with an AnteDecorator.
+
+## Contents
+
+- [`x/photon`](#xphoton)
+  - [Abstract](#abstract)
+  - [Contents](#contents)
+  - [Concepts](#concepts)
+    - [ATONE to PHOTON conversion](#atone-to-photon-conversion)
+    - [Fee enforcement](#fee-enforcement)
+  - [State](#state)
+  - [Messages](#messages)
+    - [MsgMintPhoton](#msgmintphoton)
+  - [Parameters](#parameters)
+  - [Client](#client)
+    - [gRPC](#grpc)
+    - [REST](#rest)
+  - [References](#references)
+
+## Concepts
+
+### ATONE to PHOTON conversion
+
+PHOTON is minted by burning ATONE at a conversion rate:
+
+```
+photon_minted = atone_burned * (photon_max_supply - photon_supply) / atone_supply
+```
+
+This ensures PHOTON’s supply never exceeds 1 B tokens. The module checks for
+non-zero PHOTON to mint before completing the transaction. This is because
+rounding errors can cause the calculated amount to be zero when burning small
+fractions of ATONE’s supply.
+
+### Fee enforcement
+
+An AnteDecorator ensures PHOTON (`uphoton`) is the only fee token for most
+transactions. A small set of messages (initially just `MsgMintPhoton`) can
+be set as exceptions and accept other fees such ATONE, as defined by the 
+`txfee_exceptions` parameter.
+
+## State
+
+`x/photon` stores no extra balance data, and relies on `x/bank`.
+The only tracked module data are parameters, such as whether minting is enabled.
+
+## Messages
+
+### MsgMintPhoton
+
+Burns a specified ATONE amount in exchange for newly minted PHOTON. The minted
+tokens go to the caller’s account. If `mint_disabled` is `true`, this message fails.
+
+## Parameters
+
+| Key              | Type       | Default               |
+|------------------|-----------|-----------------------|
+| mint_disabled    | bool       | false                 |
+| txfee_exceptions | []string   | ["MsgMintPhoton"]     |
+
+## Client
+
+### gRPC
+
+- Query/ConversionRate: Returns the current conversion rate.  
+- Query/Params: Returns `mint_disabled` and `txfee_exceptions`.
+
+### REST
+
+Endpoints mirror the gRPC queries, allowing retrieval of conversion rate and parameters.
+
+- `/atomone/photon/v1/conversion_rate`: Returns the current conversion rate.
+- `/atomone/photon/v1/params`: Returns `mint_disabled` and `txfee_exceptions`.
+
+## References
+
+See [ADR 002](../../docs/architecture/adr-002-photon.md) and
+[AtomOne Constitution Article 3 Section 5](https://github.com/atomone-hub/genesis/blob/b84df30364674c3f68b4bc0a43d7ed977ae22226/CONSTITUTION.md#section-5-the-photon-token)
+for more details.