feat: custom stylesheets

[alandefreitas]

This PR improves the presentation quality of the generated HTML output by fixing the two issues that have been showing up the most in recent discussions:

• Fixes Support custom stylesheets in the HTML generator #1081
• Fixes Move manual description before member list in reference documentation #1105

We’ve recently talked a lot internally about the demos not looking as polished as the project deserves (especially around HTML styling/output) and these two were the biggest offenders.

Review notes

This PR touches a lot of files, but most of the churn is not core logic:

• Lots of files are tests / expected outputs updated because the generated HTML changed.
• There are also several earlier commits that add/adjust test infrastructure so the new behavior is properly covered.

The actual implementation is much smaller and is basically in the last two commits.

Even in those last two commits, the important part to focus on is:

• template changes, and
• the source code changes

The rest is still mostly test fixtures/expected results.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (2463633) to head (f0e157a).
⚠️ Report is 12 commits behind head on develop.

Additional details and impacted files

@@       Coverage Diff       @@
##   develop   #1122   +/-   ##
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