[enhancement] bazelify docs build process

[proppy]

What's hard to do? (limit 100 words)

Currently the doc build process is defined in a recurring github workflows https://github.com/google/xls/blob/main/.github/workflows/build-mkdocs-commit-html.yml, making it difficult to troubleshoot it locally or running it from other workflows (CI).

Current best alternative workaround (limit 100 words)

Duplicate environment setup locally or in other workflows.

Your view of the "best case XLS enhancement" (limit 100 words)

Ideally the environment setup and build process would be done within bazel allowing us to share the environment setup and make the doc build process hermetic, shared and reproducible locally.