Improve Migration Guide

[joshdover]

This is an issue to track improvements that should be made to the Migration Guide and accompanying docs.

The goal of the guide is to provide all necessary guidance to successfully move a legacy plugin to the New Platform.

Improvements

• Add links to Core API docs (Improve Migration guide with examples and links #46661)
• Add table of uiExports (Improve Migration guide with examples and links #46661)
• Add more details and examples
  • "Browser-side plan of action" section
  • How to actually move code to a NP-plugin proper (last step of migration)
• Logging
  • Migration example
  • Example in TSDocs

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[mshustov]

• How to consume core types. Meaning of src/core/server/types folder to prevent such cases https://github.com/elastic/kibana/pull/47590/files#diff-f7bc3a6feccc0d692c3f2831a40194b6L8
• Recommended folder structure on the server

[rudolf]

• Fix incorrect http router shim example in MIGRATION.md (thanks @jfsiii) Update http route registration in migration guide and examples #48518

  // HTTP functionality from legacy platform, accessed in the NP convention, just like Elasticsearch 
  above.
   core.http.route({ // note: we know routes will be created on core.http
     path: '/api/demo_plugin/search',
  

[tsullivan]

https://github.com/elastic/kibana/blob/master/src/core/MIGRATION.md#configure-plugin

In one of the code sections, there is a code comment saying ... if config is optional

The guide doesn't explain what optional config means. If there is config that has default settings, would it always exist? In other words, would a config never be optional if it has a default?

[mshustov]

• Add an example of angular application usage within the new platform and what limitations it has.

[joshdover]

• Add an example of how to migrate the pre option from Hapi to New Platform router (@joshdover)

[rudolf]

• Remove comments from Migration guide that core.application.register isn't available in legacy (since Add compatibility layer for ApplicationService in legacy platform #46659)

  kibana/src/core/MIGRATION_EXAMPLES.md

  Line 464 in bd3857a

  | `chrome.setRootTemplate` / `chrome.setRootController` | -- | Use application mounting via `core.application.register` (not currently avaiable to legacy plugins). |

  kibana/src/core/MIGRATION.md

  Line 1137 in ecf8657

  | `chrome.setRootTemplate` / `chrome.setRootController` | -- | Use application mounting via `core.application.register` (not available to legacy plugins at this time). |

[joshdover]

With the majority of teams having completed their migration I am closing this issue. Whenever we focus on moving this documentation to our public docs, we should focus on reworking this doc for 3rd party plugins.