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.

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Latest commit: 9e486c8

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.47%. Comparing base (d9e6791) to head (9e486c8).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2971   +/-   ##
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  Coverage   94.47%   94.47%           
=======================================
  Files          45       45           
  Lines        7885     7885           
  Branches      708      708           
=======================================
  Hits         7449     7449           
  Misses        428      428           
  Partials        8        8           

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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! 📰