docs: add bazel sphinx_build_test replicating ReadTheDocs CI (salvage #10695)

[oharboe]

Summary

Salvage of #10695 / #10184 (merged with current master to preserve review history and fixed workspace isolation).

Adds a hermetic Bazel py_test target (//docs:sphinx_build_test) that runs the same Sphinx documentation build performed by readthedocs.org/openroad, catching broken cross-references, malformed markup, missing files, and toc.yml issues locally before they reach CI.

• //docs:sphinx_build_test runs sphinx-build -b html with pinned Python deps from @openroad-pip (no system Sphinx required).
• Tagged doc_check for filtering: bazelisk test --test_tag_filters=doc_check //docs/....

Changes from #10695 / #10184

• Preserved History: Merged origin/master into salvage-10695 keeping all commit and review history intact.
• Fixed Workspace Isolation (Review Finding): Updated docs/sphinx_build_test.py to copy docs/ and root README.md into an isolated tempfile.mkdtemp() directory before running Sphinx. This guarantees no workspace files are mutated in place and leaves no dirty git state if interrupted.
• Dependencies: Added sphinx-llm requirement (added to docs/conf.py in docs: dynamically generate llms.txt and llms-full.txt #10598) and updated bazel/requirements_lock_3_13.txt.

Fixes #9885
CC: @luarss @alokkumardalei-wq

Verification

• bazelisk test //docs:sphinx_build_test passes locally
• git status verified clean post-execution
• Signed commits (DCO)

[gemini-code-assist]

Code Review

This pull request introduces a hermetic Bazel test (sphinx_build_test) to replicate the ReadTheDocs Sphinx documentation build locally, adding the necessary Sphinx dependencies to bazel/requirements.in and updating the lockfile. Feedback on the changes suggests pinning the sphinx-llm dependency in requirements.in for reproducibility, removing the local and no-sandbox tags from the py_test target to allow sandboxed execution and caching, cleaning up an unused subprocess import, and moving the sphinx import to the top of the test script in accordance with PEP 8.