Update Charmhub home page (16)

[a-velasco]

This PR updates the metadata.yaml for PostgreSQL 16 releases on Charmhub in order to version the home page with the docs.

It addresses issue #1082

Changes

• Replaced docs: link with new RTD docs - this new Charmhub feature will create a dedicated button in the sidebar.
• Updated summary: and description: keys, since they will be the new title and content of the Charmhub home page.
• Removed old broken link to chat.charmhub.io

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.10%. Comparing base (63846ad) to head (9baaa45).
⚠️ Report is 5 commits behind head on 16/edge.

❌ Your project status has failed because the head coverage (65.10%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1111      +/-   ##
===========================================
+ Coverage    64.91%   65.10%   +0.18%     
===========================================
  Files           17       17              
  Lines         4275     4281       +6     
  Branches       656      655       -1     
===========================================
+ Hits          2775     2787      +12     
+ Misses        1333     1319      -14     
- Partials       167      175       +8     

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