Update Extension API documentation

[waylan]

Description

• Document the priorities assigned by each extension for the API Reference.
• Link Extension API docs to API Reference.
  Various sections were reduced to avoid repitition with the API Reference.
  Other sections were expanded to provide more links. A deeper explanation
  of the registry priority has been added.

Fixes 1612.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the [Documentation Style Guide](Documentation Style Guide).
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.

[waylan]

I decided that rather than writing a script which parsed the source and generated tables (as I did for the core), I would just manually create each table in the extension docs. This seems reasonable as the code is not consistent for every extension. The problem is that the manual tables are all resulting in line lengths that are too long.