Add Real-World Project: RAG Over a GitHub Repo

[abderrahim-lectures]

Add a new Real-World Projects lesson: RAG Over a GitHub Repo (backlog idea #16 — "RAG over your own GitHub repo's code + docs").

Scope

• docs/projects/github-repo-rag/index.md — new lesson matching the house skeleton (see docs/projects/rag-notes/index.md as the closest sibling in structure: local embeddings, similarity search, free-tier LLM). Builds on the RAG App / rag-notes project.
• examples/github-repo-rag/ — a real, runnable tool: clones (or points at) a local repo folder, chunks code + Markdown docs, embeds them locally with sentence-transformers, and answers questions over the repo with a free-tier LLM, citing the file/line each answer comes from. Includes a query.py, a sample small repo to clone for testing, a notebook.ipynb, and a README.md.
• Registration: src/data/projects.ts, docs/projects/index.mdx, src/pages/index.tsx (English only — no i18n changes).
• Colab/Kaggle/Binder badges in "Where to run this", pointed at this PR's branch (to be updated to main once merged) — the notebook clones a small real public repo (e.g. this course's own repo or a small sample repo) and runs the pipeline over it, asking for the API key interactively with getpass.

Framing note

The lesson should be honest about chunking code vs. prose (code chunks need line/function boundaries, not just character windows) and about retrieval being approximate: it should encourage spot-checking that cited files actually exist and that the answer is grounded in them.