docs: update stale documentation to reflect recent codebase changes
- Mark cog train as deprecated (was still labeled experimental)
- Add COG_SETUP_TIMEOUT and COG_WEIGHTS env vars to environment.md
- Rewrite deploy.md to reference coglet, cog serve, health checks, concurrency
- Add GET / root discovery endpoint to http.md
- Fix http.md concurrency claim (said single prediction, concurrency exists)
- Fix cancel example in http.md to use BasePredictor class style
- Fix Python version inconsistencies (3.12 → 3.13 to match cog.yaml)
- Add Homebrew install option to getting-started-own-model.md
- Add missing Input() args (min_length, max_length, regex, deprecated)
- Mark cog.File as deprecated in getting-started-own-model.md
- Remove pinned jupyterlab 3.3.4 in notebooks.md
- Add HTTP API redirect link to redis.md tombstone
- Regenerate llms.txt and cli.md
