Fix outdated documentation and incorrect docstrings

[mfosterw]

• Update docs/webiscite.rst to reflect current architecture: GithubWebhookView and PullRequestHandler replace the removed github_hook, process_pull, and pr_opened identifiers
• Fix managers.py docstrings: correct parameter names and return descriptions for create_from_github on both managers; add missing docstrings to get_queryset and annotate_user_vote; add module docstring
• Fix webhooks.py docstrings: update module docstring to class-based view; fix _validate_header and _validate_signature return type descriptions
• Add missing module docstring to activitypub/models.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c9ce2c9) to head (ec2d11a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #293   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           49        49           
  Lines         2009      2009           
=========================================
  Hits          2009      2009           

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