[2.x] Strip Markdown from sidebar table of contents

[emmadesilva]

Fixes #2591.

Summary

• convert Markdown heading titles to plain text before displaying them in the sidebar table of contents
• preserve the existing heading identifier used by the table of contents
• add a regression test using the linked-heading example from the issue

Testing

• php vendor/bin/phpunit packages/framework/tests/Unit/GeneratesSidebarTableOfContentsTest.php
• php vendor/bin/phpunit packages/framework/tests/Feature/Views/SidebarTableOfContentsViewTest.php
• php vendor/bin/phpstan analyse packages/framework/src/Framework/Actions/GeneratesTableOfContents.php packages/framework/tests/Unit/GeneratesSidebarTableOfContentsTest.php --no-progress
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4211c85) to head (c02700d).
⚠️ Report is 4 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff             @@
##                 2.x     #2607   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity      1628      1628           
===========================================
  Files            169       169           
  Lines           4097      4097           
===========================================
  Hits            4097      4097           

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