Add GitHub connector docs and config example (MAIT-92)

[acikabubo]

Summary

• Add GitHub connector example to `config.yaml.example` with all options: PAT/GitHub App auth, branch/commit_sha targeting, file extension and directory filters, issue ingestion with label filters, `concurrent_requests`
• Move GitHub from "Extra connectors" to "Connectors included" in README
• Add GitHub Connector documentation section with `config.yaml` and `.env.rag` examples, including GitHub App auth alternative
• Add GitHub connector ENV vars to `.env.rag.example`

Test plan

• Review `config.yaml.example` — GitHub connector block is commented out and all options documented
• Review README GitHub Connector section for accuracy
• Review `.env.rag.example` — GitHub connector ENV vars present

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