add remark-insert-headings to list of plugins

[tgreyuk]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Added remark-insert-headings to list of plugins. This was initially created to solve a problem with integrating remark-toc into an auto documentation tool but could also serve a wider purpose.

https://github.com/tgreyuk/remark-insert-headings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ed7b185) to head (dfdf5a1).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1461   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          138       138           
=========================================
  Hits           138       138           

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