Add docs for `footnoteLabelTagName`, `footnoteLabelProperties`

[gtnbssn]

This is to keep the doc in sync with mdast-util-to-hast.

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

Copied the content from new content from the current mdast-util-to-hast readme.md.

[codecov-commenter]

Codecov Report

Merging #24 (5755df6) into main (21ee7e7) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #24   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           79        79           
=========================================
  Hits            79        79           

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[github-actions[bot]]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.

[wooorm]

Thanks! :)