Build: remove docusaurus configuration files

[lukegalbraithrussell]

This PR removes the docusaurus build from this repo, moving to a centralized setup. Markdown files only from now on in the open source repos.

This PR should not be merged until these docs are live on docs.slack.dev/java-slack-sdk. (I'm specifying this build-docs-remove-docusaurus branch in the initial build to prevent downtime).

This will take place at the same time as all the other tools repos migrations

Category (place an x in each of the [ ])

just docs!!!

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.87%. Comparing base (cf51111) to head (94d3b93).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1491   +/-   ##
=========================================
  Coverage     72.87%   72.87%           
  Complexity     4372     4372           
=========================================
  Files           475      475           
  Lines         14244    14244           
  Branches       1447     1447           
=========================================
  Hits          10380    10380           
  Misses         3008     3008           
  Partials        856      856           

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