Document the new in-repo examples directory

[Copilot]

Cog’s example models now live in replicate/cog under examples/, but the README and public docs still pointed at the old standalone examples repository. This updates the docs surface area to treat the in-repo examples tree as the canonical entry point.

• README + docs entry points

  • Replaced the old replicate/cog-examples link in the root README with the new in-repo examples docs page.
  • Added examples references to the getting started and deployment docs so example projects are discoverable from common entry paths.

• New public examples page

  • Added docs/examples.md and wired it into mkdocs.yml.
  • Summarizes the main examples by use case: minimal runners, image models, training, streaming, context, secrets, notebooks, and managed weights.

• Examples index in the repo

  • Added examples/README.md as a root index for the examples tree.
  • Includes short, scan-friendly descriptions for each example, plus the experimental managed-weights example.

• Generated docs sync

  • Regenerated docs/llms.txt so the bundled public docs reflect the new examples location and links.

Example of the updated docs entry point:

- [Browse the example models in this repo](docs/examples.md)