✨ FontAwesome v6 roles, CDN config, fontawesome5 LaTeX support

[chrisjsewell]

Modernizes the FontAwesome integration around three declarative config options.

Changes

• v6 roles: fa-solid / fa-brands / fa-regular, emitting exactly those classes. The legacy fa/fas/fab/far roles are unchanged (and documented as legacy). This is the supported path for FontAwesome Pro kits (FontAwesome import interfering with Pro License #174): load your kit, use v6 names.
• sd_fontawesome_source ("none" default / "cdn") + sd_fontawesome_cdn_url: opt-in CDN loading via app.add_css_file, replacing the docs' copy-paste html_css_files instruction. The docs now dogfood it.
• sd_fontawesome_latex grows string values while keeping True/False working (True ≡ "fontawesome", output verified byte-identical to main): "fontawesome5" loads fontawesome5.sty and emits \faIcon{name} / \faIcon[regular]{name} (brands resolve by bare name — mapping independently verified against the package's completion data). This resolves the Latex build broken in Sphinx 8 #242 class of clash: if your theme loads fontawesome5, set the same value so both agree. The stored value stays raw (TOML-round-trippable); normalization is a computed property.
• Per-icon "not included in output" warnings now fire once per build (once=True, verified on the 7.2 floor).
• Docs: FA section rewritten around the config, with an explicit Pro-kits subsection, a legacy-names note, the LaTeX value table, and a caveat that the LaTeX packages resolve FA5-era icon names (v6-renamed icons render in HTML but not in PDF).

Verification

• Independent review: no correctness findings. Verified empirically: incremental rebuilds propagate config flips to unrebuilt pages (rebuild="env"); absolute CDN URLs are emitted verbatim (no ?v= mangling, no copy attempt) on 7.2.6 and 9.x; epub with "cdn" behaves identically to the pre-existing html_css_files approach; bool|str confval type-checking clean across the range; no duplicate CDN link in the dogfooding docs build.
• Legacy parity locked by test: sd_fontawesome_latex = True produces the same \usepackage{fontawesome} + \faicon{...} output as main.
• Full suite on pinned Sphinx 7.2.6 / 7.4.7 / 8.2.3 (184 passed each); new tests/test_fontawesome.py covers roles, CDN link exactly-once/absent, both LaTeX packages, and once-per-build warnings.

Closes #174
Closes #242

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.44%. Comparing base (abbf459) to head (d53f19b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #285      +/-   ##
==========================================
+ Coverage   90.72%   91.44%   +0.72%     
==========================================
  Files          13       13              
  Lines        1207     1251      +44     
==========================================
+ Hits         1095     1144      +49     
+ Misses        112      107       -5     

Flag	Coverage Δ
pytests	91.44% <100.00%> (+0.72%)	⬆️

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