Add a doc source for a new library: Django

[AshwinUgale]

Why

Follow-up to #11: grow the set of libraries DocChat can answer for. Same registry pattern, different source repo.

What to do

• Add a _LibraryConfig entry for Django (django/django, docs under docs/) in sidecar/src/docchat_sidecar/indexer.py.
• Note: Django's docs are reStructuredText (.rst), not Markdown - so this one likely needs a small .rst noise-stripping tweak vs. the current Markdown path. Call that out in the PR.

Acceptance criteria

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

Pointers

• indexer.py: _LibraryConfig, _LIBRARY_CONFIG, the chunking/noise-stripping helpers.