docs: add bazel sphinx_build_test replicating ReadTheDocs CI

Adds a hermetic Bazel `py_test` target 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/...`.
- Mirrors conf.py setup (main symlink, README2.md prefix swap, messages
  glossary generation, doxygen stub) and strips `bazel-*` symlinks to
  prevent recursive scanning.

Fixes #9885

Signed-off-by: alokkumardalei-wq <alokkumardalei2@gmail.com>

Author: alokkumardalei-wq

bazel/requirements.in

@@ -1 +1,11 @@
 tclint==0.7.0
+
+# Sphinx documentation build (replicates ReadTheDocs CI check)
+# See: https://github.com/The-OpenROAD-Project/OpenROAD/issues/9885
+sphinx
+sphinx-book-theme
+sphinx-copybutton
+sphinx-external-toc==0.3.1
+sphinx-tabs
+sphinxcontrib-mermaid
+myst-parser