✨ Add sd_fontawesome_version for version-agnostic icon roles

[chrisjsewell]

Decouples FontAwesome role names from the CSS class scheme they emit, so users never touch their documents when changing FA versions — the follow-up agreed in #285's review discussion.

Design

New TOML-representable option:

sd_fontawesome_version = "as-named"   # or "4" | "5" | "6"

• "as-named" (default): today's exact behaviour — the role name is emitted verbatim as the leading class. Byte-parity with main proven by sha256 over a build exercising all seven role spellings.
• A version value: any role spelling becomes a pure style selector and emits that version's class scheme — :fas:rocket and `:fa-solid:`rocket both produce fa-solid fa-rocket under "6", fas fa-rocket under "5", fa fa-rocket under "4" (v4 had no style prefixes; bare fa maps to solid under 5/6). Upgrading FontAwesome is then a one-line conf.py change; with "6", the concise spellings also work with Pro kits.
• LaTeX output is fully version-independent: the semantic style now travels on the node (icon_style attribute), so fontawesome5 rendering (\faIcon[regular]{…} etc.) is identical for every version value — including "4", whose HTML-side style collapse no longer leaks into PDFs (a review finding, fixed by storing the style rather than re-deriving it from the emitted class).
• Docs lead with the version-agnostic story; the icon-name churn caveat stays (renames across FA versions are emitted as written, HTML and LaTeX alike).

Verification

• Independent review (merge-safe): as-named byte-parity independently reproduced (identical sha256 across main / branch-default / branch-explicit, HTML and .tex); config flip verified to re-read all documents on incremental builds (rebuild="env" — parse-time class baking is safe); extra-classes microsyntax preserved under translation; nested contexts (buttons/cards/dropdowns) clean; parallel -j4 reads consistent; invalid value warns and falls back.
• LaTeX invariance now locked by a test parametrized over all four version values.
• Full suite on pinned Sphinx 7.2.6 / 7.4.7 / 8.2.3 (246–249 passed per revision); the only fixture change is the icon_style attribute appearing on fontawesome nodes (audited).

Follow-up to #285 / #174

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.63%. Comparing base (58eca26) to head (8f6796e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
+ Coverage   92.60%   92.63%   +0.03%     
==========================================
  Files          13       13              
  Lines        1365     1371       +6     
==========================================
+ Hits         1264     1270       +6     
  Misses        101      101              

Flag	Coverage Δ
pytests	92.63% <100.00%> (+0.03%)	⬆️

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