docs: removing external links for unified nav

[haleychaas]

I'm working on uniting the nav bar for all of the tools so that you don't lose the context of the rest of the nav when you click on one of the tools. In this PR, I'm removing the external links (release notes, github repo, contributor's guide) to cut down on bloat in the nav menu when they are all united. Since the github repo and contributor's guide are already mentioned in the landing page, I just added the release notes link to that.

Preview for one nav here: https://docs-slack-d-unified-na-idfrpr.herokuapp.com/tools
PR for unified nav here: https://github.com/slackapi/docs/pull/605

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

• 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 73.30%. Comparing base (b6586d9) to head (ac43adb).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1603      +/-   ##
============================================
- Coverage     73.33%   73.30%   -0.03%     
+ Complexity     4521     4519       -2     
============================================
  Files           478      478              
  Lines         14300    14300              
  Branches       1490     1490              
============================================
- Hits          10487    10483       -4     
- Misses         2922     2930       +8     
+ Partials        891      887       -4     

Flag	Coverage Δ
jdk-14	73.30% <ø> (-0.03%)	⬇️

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[zimeg]

@haleychaas LGTM! Same ordering suggestion as the Bolt for Python PR - is moving "Release notes" before "Contributing" alright for a first read?

• Getting help
• Release notes
• Contributing

Feel free to merge whenever best! 📰