Implement Dokka API reference

[aSemy]

This PR updates the build config to generates Dokka HTML API code reference for all subprojects.

To generate the API docs:

1. Run ./gradlew :docs:dokkaGen
2. Open docs/build/dokka/html/index.html in a browser.

Part of #2228.

The next step is publishing the API reference to a website (e.g. GitHub Pages).

Screenshots

Here are some example screenshots.

• The home page shows a list of all modules, each with a brief description (from the README.md in each subproject)

  

• When viewing a module, all content from the module's README.md is available

  

• Code and KDoc are easily viewable and searchable
  

Summary

• Update subproject READMEs to use as Dokka Module files (add # Module header)
• Generate aggregated Dokka HTML in the :docs subproject
• Remove 'archived' from headings - Dokka needs the heading to match the module name exactly. Instead, use admonitions. But... Dokka doesn't support them. So instead, find/replace the admonitions with emojis.

Reasons for using Dokkatoo over the existing Dokka Gradle Plugin.

• Dokkatoo compatible with the Gradle API and follows best practices, resulting in better performance. DGP is not compatible, resulting in worse build performance and more Gradle warnings.
• kotlin-wrappers has a :docs subproject. It makes sense to aggregate the API docs into this subproject. DGP cannot create an aggregated HTML doc into a subproject, while Dokkatoo can.