Tracker: build docs without heavy deps (mock imports) & page-level HTML build cache

[mishig25]

Tracker for the two build-speed workstreams.

Mock imports: build docs without installing heavy dependencies

• Add --mock_deps: build docs without installing heavy dependencies #801 — mock heavy imports (torch, tensorflow, flax, jax, ...) so doc builds only need the documented library + light deps
  • Mock system: meta-path finder passing find_spec/importlib.metadata availability checks, signature-safe subclassing (PEP 560), decorator pass-through, dotted-path reprs, tuple-comparable __version__, issubclass/isinstance answering False
  • Per-library registry mock_deps/<library>.txt applied automatically by build/preview (replaces the --mock_deps flag syntax; flag kept as additive extra)
  • real:/nodeps: registry lines + doc-builder light-install (bare names pinned from the library's own metadata)
  • Shared workflows: light-install path, custom_container input removed, uv-only (setup-uv, uvx for hf sync)
  • Validated in fresh venvs: all 25 builds green (18 libraries + the optimum family, incl. transformers 723 pages byte-identical to prod on bert/llama)

Page-level HTML build cache

• Page-level HTML build cache backed by HF storage buckets #800 — reuse prerendered page HTML from a Hugging Face storage bucket (hf://buckets/hf-doc-build/doc-build-cache) so a one-file PR doesn't rebuild hundreds of pages
  • Cache key: version-normalized generated MDX + kit tree hash + doc-builder version; cross-version reuse rewrites page URLs while assets stay pinned to the hosted source version
  • Trusted main builds write (--html_page_cache_write), PR builds read-only (cache-poisoning prevention); cache failures degrade to a normal build, never fatal