feat: support .mailmap for git co-authors and author exclusions in au…

[luminoso]

Git applies .mailmap to %aN and %aE author fields, but leaves the commit message body (%B) untouched. Because mkdocs-document-dates parses Co-Authored-By: trailers directly from the body text, co-authors bypass .mailmap rules.

This pull request resolves co-authors through git check-mailmap, adds author exclusion support to authors.yml, and prevents duplicate author avatars on the rendered page.

What changed

• Mailmap resolution for co-authors (utils.py): parse_commit_authors now passes extracted Co-Authored-By: pairs through git check-mailmap, mapping variations and aliases to their canonical name and email.
• Flexible authors.yml lookup (plugin.py): _load_authors_from_yaml indexes entries by dictionary key, configured name, and email so metadata attaches regardless of which identifier git returns.
• Author exclusion flag (plugin.py): setting ignore: true or exclude: true on an author in authors.yml drops them from the rendered footer. This lets sites omit bots and service accounts without disabling git author tracking.
• Author deduplication (plugin.py): _load_author_cached deduplicates entries by canonical author name, avoiding duplicate avatars when multiple aliases appear in history.

How to test

1. Add a .mailmap entry pointing alias co-authors to a single identity:

   Jane Doe <jane@example.com> <jane.dev@example.com>
   

2. Commit a markdown file with Co-Authored-By: Jane Dev <jane.dev@example.com>.
3. Build the site with mkdocs build.
4. Check the rendered HTML footer: the page displays Jane Doe <jane@example.com> instead of the raw trailer value.
5. Add ignore: true to an author in authors.yml and rebuild: the author is omitted from the page footer.