feat: Increase indent of figures

[martinthomson]

This carries some risk, but on balance, I think that it is a good change.

We see a lot of documents that include sourcecode or ASCII art that appears at the same indent level as the rest of the text. This makes it hard to visually distinguish figures.

This increases the indent for these, where possible. It retains the indentation flexbox hack that ensures that wider figures reduce the margin.

The risk is that this will double the indent where people have manually added one. The text formatter of xml2rfc doesn't add an indent, so this is also an inconsistency in appearance. My hope is that xml2rfc will be updated to match, with people learning not to add their own indentation to documents.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.70%. Comparing base (18cea10) to head (526d747).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8323      +/-   ##
==========================================
- Coverage   88.71%   88.70%   -0.02%     
==========================================
  Files         320      320              
  Lines       41793    41788       -5     
==========================================
- Hits        37078    37068      -10     
- Misses       4715     4720       +5     

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