Move Kibana Platform documentation to Kibana's Developer Guide

[joshdover]

In support of #41833 and preparation for more external communications of the Kibana Platform, we should move our documentation into asciidoc. This first pass is focused only on handwritten documentation and not the generated API docs.

Docs to move:

• src/core/MIGRATION.md
• src/core/MIGRATION_EXAMPLES.md
• src/core/CONVENTIONS.md
• src/core/TESTING.md

Docs to remain:

• src/core/CORE_CONVENTIONS.md
• src/core/PRINCIPLES.md

I propose we create two sections under the "Developer guide" section: one for legacy plugins, one for platform plugins. We can include a high-level overview of the platform as well as these existing docs on how to migrate to it.

[elasticmachine]

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

[mshustov]

I suspect PRINCIPLES.md remains the internal documentation. Doesn't it?

[joshdover]

I suspect PRINCIPLES.md remains the internal documentation. Doesn't it?

Ah yes, missed that one. Updated the description.

[joshdover]

One concern that came up was the docs on elastic.co being out of sync with the repository. It turns out the docs are rebuilt and published every 30 minutes, so I do not think this is much of a concern.

[stacey-gammon]

@lukeelmers - should we close this? Legacy plugins aren't really a thing anymore.

[pgayvallet]

Is that a dupe of #57343?

[stacey-gammon]

I changed the title yesterday, originally this issue specifically mentioned new platform (hence migrations, and migration examples - not saved object migrations but migrating from old platform to new platform). So I say we close as obsolete, but closing as a dup works fine for me too :D

[pgayvallet]

Closing as Math.rand() > 0.5 ? "obsolete" : "duplicate"