Fix up releasing workflow and docs

[gadomski]

Description:

I didn't get the release flow quite right for https://github.com/stac-utils/pystac/releases/tag/v1.7.0, so this fixes up the docs for next time, and tweaks the workflow to run on publish (and not just created, since created only fires if you make a new tag).

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.34%. Comparing base (50939ee) to head (cb994c7).
Report is 274 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1032   +/-   ##
=======================================
  Coverage   90.34%   90.34%           
=======================================
  Files          47       47           
  Lines        6213     6213           
  Branches      929      929           
=======================================
  Hits         5613     5613           
  Misses        422      422           
  Partials      178      178           

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