Add a doc source for a new library: Flask

[AshwinUgale]

Why

The indexer only knows three libraries (react, fastapi, vue). Each new library is a self-contained registry entry, making this a great repeatable contribution. Flask is a natural next pick: the Pallets docs live in the same repo as the source, so version tags resolve cleanly - the same story as FastAPI.

What to do

• In sidecar/src/docchat_sidecar/indexer.py, add a _FLASK_DOC_PATHS tuple of the most useful Markdown doc paths from pallets/flask, then register a _LibraryConfig entry in _LIBRARY_CONFIG with an appropriate ref_for (version tag vs. main).
• Confirm the fetched files chunk cleanly (frontmatter/import stripping is already handled for MDX; adjust if Flask's docs need different noise-stripping).

Acceptance criteria

• The library resolves through _urls_for and indexes without error against a mocked fetch (extend sidecar/tests/test_indexer.py).
• Ideally add 2-4 Q/A pairs for the new library to evals/corpus.json.

Pointers

• indexer.py: _LibraryConfig, the _LIBRARY_CONFIG registry, and the _*_DOC_PATHS tuples.

The same recipe applies to Django (#12), Express, Svelte, Pydantic, etc. - each is independently pickable.