Restore the contrasting version in the docs title

[timkpaine]

Themes reuse html_title for the <title> tag, which is why the styled version span was dropped in 2d69e4a: the markup showed up escaped in browser tabs.

Add HtmlTitle, a str whose html returns the tag-stripped text. Sphinx renders docstitle raw in the theme header but pipes it through |e or |striptags|e for <title>, and both filters go via html, so one value gives markup in the header and plain text in the tab.

html_title itself stays a plain str, since check_confval_types compares type(value) exactly and rejects str subclasses, and a class defined in the generated conf.py cannot be pickled with the build environment. The styled form is injected as context["docstitle"] instead, skipping the search page, which is the one template that interpolates docstitle unfiltered.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.20%. Comparing base (4aa5ce1) to head (a167048).

Files with missing lines	Patch %	Lines
yardang/utils.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #157      +/-   ##
==========================================
- Coverage   86.22%   86.20%   -0.03%     
==========================================
  Files          16       16              
  Lines        2026     2030       +4     
  Branches      209      209              
==========================================
+ Hits         1747     1750       +3     
- Misses        217      218       +1     
  Partials       62       62              

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