Document markdown feature files

[cressie176]

Summary

• Adds a Markdown Feature Files section to docs/feature-files.md documenting the MarkdownFeatureParser syntax: headings for constructs, list-item steps, fenced doc-strings, GitHub tables, annotations, and the > / <!-- --> comment forms.
• Covers entity decoding (prose vs. verbatim doc-strings) and the inline-code-span limitation.
• Updates the README "Why Yadda?" bullet to link the new docs section instead of the example.

Follows #353.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.35%. Comparing base (98b615e) to head (234b056).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #354   +/-   ##
=======================================
  Coverage   87.35%   87.35%           
=======================================
  Files          66       66           
  Lines        2625     2625           
=======================================
  Hits         2293     2293           
  Misses        332      332           

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