feat: Re-add recentupdate directive

[SilverRainZ]

Summary

• Add recentupdate directive using BaseContextDirective from sphinxnotes-render
• Extract collect_revisions() shared helper for directive and extra context
• Restore recentupdate_template config value as default template when directive body is empty

Directive Usage

.. recentupdate:: [count]
   :self:
   :paths:
      docs/changelog.rst

   custom jinja template

• Argument count: optional, defaults to recentupdate_count config
• Option :self:: only revisions affecting current document
• Option :paths:: git pathspecs, one per line
• Body: Jinja template with revisions context, falls back to recentupdate_template

Co-authored-by: MiMoCode mimo@xiaomi.com