docs: shared PyAuto brand layer for the Furo docs

[Jammy2211]

Summary

Phase B of the docs middle path (PyAutoLabs/pyautolabs.github.io#1; census + decision record #1341). Adds the stack-wide docs/_static/pyauto.css — identical file in PyAutoFit/PyAutoGalaxy/PyAutoLens: the PyAuto family gradient strip across the top of every docs page (the three stack accents) and sidebar wordmark styling — plus this project's own Furo accent (amber #c2410c light / #f28c38 dark) via light_css_variables/dark_css_variables. Also lifts the .gitignore docs/_static entries (the reason no static asset could ever be committed here).

Pairs with the new front door at https://pyautolabs.github.io (phase C, same palette). Zero-cost per the decision record: *.readthedocs.io stays canonical.

API Changes

None — docs styling only.

Test Plan

• Suite: 1424 passed, 14 skipped (full, worktree)
• Sphinx build: warning count unchanged at the baseline (67); pyauto.css deployed into _static, brand variables present in rendered HTML
• docs CI check (phase A) green on this PR

Validation checklist (--auto run)

• Effective level: supervised (header: supervised, cap: docs ≤ medium → safe)
• Plan: on the issue (pyautolabs.github.io#1), written at launch (human directive: "do everything else you can now, then stop for the night")
• Gate: tests 1424 passed, 14 skipped · smoke n/a (docs styling only) · review CLEAN · Heart YELLOW — reason set verified identical to the set the human acked in-session earlier tonight (rtd-hygiene ship); treated as launch-time ack for this launch, disclosed here for validation
• PyAutoFit leg only: EP worktree claims overridden per the human's in-session direction earlier tonight ("they don't clash"); file disjointness re-verified
• Human: plan sound in hindsight?
• Human: diff matches plan (no scope creep)?
• Human: merge, amend, or reject — then log the outcome

Full API Changes (for automation & release notes)

None — files touched: docs/_static/pyauto.css (new), docs/conf.py, `.gitignore`.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GziFkivBHz1d11eFYUszmy