Add --local-theme flag to breeze build-docs for local theme development

[srchilukoori]

Add a --local-theme flag to breeze build-docs that bind-mounts a local
sphinx-airflow-theme directory into the container. This lets developers
iterate on theme changes without publishing a wheel or bumping versions.

Usage:

breeze build-docs --local-theme ~/airflow-site/sphinx_airflow_theme

Follows the existing --mount-ui-dist compose overlay pattern: one new
compose file, one ShellParams field, one click option. Default behavior
(no flag) is unchanged; CI never passes --local-theme.

This replaces the approach in #65840 (adding sphinx-airflow-theme as a uv
workspace member), which was closed after the reviewer pivoted to a
bind-mount approach.

closes: #45576
related: #65840

Files changed

File	Change
scripts/ci/docker-compose/mount-local-theme.yml	Compose overlay, mounts $LOCAL_THEME_PATH over /opt/airflow/docs-theme/sphinx_airflow_theme/
dev/breeze/src/airflow_breeze/utils/path_utils.py	Path constant
dev/breeze/src/airflow_breeze/commands/common_options.py	--local-theme click option (Path, dir-only, resolve)
dev/breeze/src/airflow_breeze/params/shell_params.py	Field, compose overlay append, env var
dev/breeze/src/airflow_breeze/commands/developer_commands.py	Wire through build_docs and _build_python_docs
dev/breeze/src/airflow_breeze/commands/developer_commands_config.py	Help grouping
dev/breeze/tests/test_shell_params.py	4 test cases (env var + compose overlay)
dev/breeze/doc/images/output_build-docs.*	Auto-regenerated CLI help

Test plan

• pytest dev/breeze/tests/test_shell_params.py (20 passed)
• pytest dev/breeze/tests/test_docker_command_utils.py dev/breeze/tests/test_run_test_args.py (86 passed)
• prek run --from-ref upstream/main --stage pre-commit (all passed)
• Manual: breeze build-docs --local-theme <path> with a local airflow-site checkout

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Was generative AI tooling used to co-author this PR?

• Yes (Claude)

[potiuk]

I think we also need a doc describing the process. It also should run doc build to automatically rebuild changes (The docs build already has this option) and we should make sure that the mounted sphinx_theme files are automatically rebuilt when they are changed in the mounted directory - this should be locally tested if ti works as expected