Add remark-directive-mdx plugin entry

[re-xyr]

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

remark-directive-mdx turns directives parsed by remark-directive into JSX elements used by MDX. This change adds remark-directive-mdx to the plugins list.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ed7b185) to head (271abe8).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1467   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          138       138           
=========================================
  Hits           138       138           

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[remcohaszing]

We list MDX specific plugins in the MDX plugin list, not in the remark plugin list. IMO we should approve mdx-js/mdx#2664, but reject this PR.