[FLINK-39067][docs] Move Python documentation from Hugo to PyDocs

[MartijnVisser]

What is the purpose of the change

Migrate Python-specific documentation pages from the main Hugo docs site into the Sphinx-based PyDocs (flink-python/docs/) as part of FLIP-561. This consolidates all Python documentation into a single cohesive location.

Brief change log

• Add user_guide/ section to PyDocs covering Table API, DataStream API, configuration, dependency management, debugging, metrics, FAQs, and Python REPL
• Replace moved Hugo pages (English + Chinese) with redirect stubs to preserve backward-compatible URLs
• Set up Sphinx i18n with gettext for Chinese translations, including populated .po files from existing Hugo Chinese content
• Add language switcher component to PyDocs navbar
• Use sphinx.ext.extlinks with FLINK_DOCS_BASE_URL env var for version-configurable cross-references to main Flink docs
• Update CI docs.sh to build Chinese PyDocs into zh/ subdirectory
• Add sphinx-intl to sphinx dependency group in pyproject.toml
• Add Makefile targets for gettext and Chinese docs builds
• Update .gitignore for locale build artifacts

Verifying this change

PyDocs (English + Chinese)

  cd flink-python
  pip install -e ".[sphinx]"
  cd docs
  make clean && make html-zh && make serve

Then open http://localhost:8080/ and check:

• "User Guide" appears in the left nav
• All pages render correctly (code blocks, tables, links)
• :flinkdoc: links point to flink-docs-master
• Language toggle in the top navbar switches between English and Chinese (/zh/)

Hugo docs (redirects)

  cd docs
  git submodule update --init --recursive
  ./setup_hugo.sh
  ./setup_docs.sh
  ./build_docs.sh --skip-integrate-connector-docs

Then at http://localhost:1313/ check:

• "Python" section is gone from left nav under Flink APIs
• "Python DataStream" section is gone under DataStream API
• "Python REPL" is gone under Deployment > REPLs
• "Python User-defined Functions" still appears under Table API > Functions (as a stub with a link to PyDocs)
• Old URLs like /docs/dev/python/ redirect to /api/python/user_guide/

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• d7a1fd5 Azure: FAILURE

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