Update Point Cloud Extension

[duckontheweb]

Related Issue(s):

• Update extension pydoc strings to match new extension architecture #326
• Implement collection summary functionality #327

I've intentionally left #518 for a separate PR that includes all extension name changes.

Description:

• Updates docstrings and adds Collection summaries for Point Cloud Extension

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

❌ Patch coverage is 96.24060% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.50%. Comparing base (77f203e) to head (b31cd77).
⚠️ Report is 887 commits behind head on main.

Files with missing lines	Patch %	Lines
pystac/extensions/pointcloud.py	93.82%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #558      +/-   ##
==========================================
+ Coverage   94.43%   94.50%   +0.06%     
==========================================
  Files          71       71              
  Lines       10611    10703      +92     
  Branches     1092     1095       +3     
==========================================
+ Hits        10021    10115      +94     
+ Misses        420      418       -2     
  Partials      170      170              

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