Compact the documentation UI

[polishdataengineer]

What changed

• reduced the home-page hero from an oversized landing block to a compact documentation header
• simplified the primary actions and moved migration/benchmark links into a lighter secondary row
• turned the LLM controls into a compact, accessible navigation toolbar placed after the page heading/hero
• grouped sidebar navigation into Start, Guides, Reference, and Project
• disabled Furo's 110% ultra-wide scaling and tightened sidebar/TOC spacing
• fixed custom surface colors when Furo uses automatic dark mode
• regenerated the LLM documentation bundles

Why

At wide desktop sizes the hero title reached about 76 px, the hero grew beyond 500 px, and a second large toolbar duplicated several actions. The first viewport was dominated by chrome instead of documentation content.

Impact

At 1440×900 the hero is now 280 px and the LLM toolbar 47 px; the Why Testenix section starts in the first viewport. The layout has no horizontal overflow at 1440, 1024, 390, or 320 px, and interactive targets remain at least 36 px high.

Validation

• python scripts/generate_docs_assets.py --check
• sphinx-build -W --keep-going -b dirhtml docs site
• node --check docs/_static/copy-llm.js
• pytest -q — 201 passed
• ruff check .
• ruff format --check .
• browser checks at 2048×1152, 1440×900, 1024×768, 390×844, and 320×700
• light and automatic dark themes checked; no console errors
• both LLM copy actions verified