Parallelize docs builds with sphinx-build -j

[GarrettBeatty]

Description of changes:

Part 1 of 3 (stacked) of the CI build-perf changes from #10759, targeting feature/build-perf. Speeds up the docs build (the Catapult BuildDocs step).

sphinx-build -j N barely helped before because the guzzle theme was not marked parallel-safe and because Sphinx does most of the parallel work in its main process for a doc set this size (~20k pages):

• guzzle_sphinx_theme: declare the theme parallel-safe, and build sitemap.xml from the environment (plus pages written by the main process) so it is complete and ordered like a serial build when pages are written in worker processes.
• parallel_chunks (new extension): while reading, use one interleaved chunk per process. Sphinx otherwise creates ~180 small chunks and the main process unpickles each worker's entire environment, which grows with every chunk. Also restores the serial document order after reading, since it leaks into searchindex.js.
• parallel_write (new extension): load, resolve, post-transform and index doctrees in the worker processes instead of the main process, and merge the search index fragments and images back in chunk order. This is the approach from Parallel post transform including write_doc_serialized sphinx-doc/sphinx#11746, specialized for the HTML builder.

Both extensions override private Sphinx methods, so they need to be re-validated when upgrading Sphinx (6.2.0 is pinned in requirements-docs-lock.txt).

Testing:

Measured on a c5.9xlarge (same vCPU/memory as CodeBuild BUILD_GENERAL1_XLARGE): 56.6 min (serial) → 9.4 min with -j 16.

• Output is identical to a serial build: all 40,916 files are byte-identical except .buildinfo (which hashes the config), including searchindex.js and sitemap.xml; same warning count.
• End-to-end dry run in a Catapult dev stack: BuildDocs 49.5 → 9.8 min compared with prod medians.

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