docs: removing external links for unified nav

[haleychaas]

Summary

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

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.88%. Comparing base (2d370dc) to head (c147b87).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2625   +/-   ##
=======================================
  Coverage   88.88%   88.88%           
=======================================
  Files          62       62           
  Lines       10238    10238           
  Branches      451      451           
=======================================
  Hits         9100     9100           
  Misses       1117     1117           
  Partials       21       21           

Flag	Coverage Δ
cli-hooks	88.88% <ø> (ø)
cli-test	88.88% <ø> (ø)
logger	88.88% <ø> (ø)
oauth	88.88% <ø> (ø)
socket-mode	88.88% <ø> (ø)
web-api	88.88% <ø> (ø)
webhook	88.88% <ø> (ø)

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[changeset-bot]

⚠️ No Changeset found

Latest commit: c147b87

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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! 📰