Docs/semantic headings

[jonathanbossenger]

While working on the 6.9 Call for testing post, I realised some of the doc headings are not using semantic markup, making it harder to link to specific sections in the docs. This PR fixes that by applying the correct heading markup

[github-actions]

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Co-authored-by: jonathanbossenger <psykro@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.69%. Comparing base (cbe8a59) to head (653d8f3).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk      #96   +/-   ##
=========================================
  Coverage     85.69%   85.69%           
  Complexity      103      103           
=========================================
  Files            16       16           
  Lines           776      776           
  Branches         86       85    -1     
=========================================
  Hits            665      665           
  Misses          111      111           

Flag	Coverage Δ
javascript	92.66% <ø> (ø)
unit	82.97% <ø> (ø)

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[gziolo]

Looks good to me 👍🏻