Link to the docs to get started build an extension

[rajbos]

Nothing in the readme points to the documentation of setting up all the moving parts for an Extension. List the steps in the readme and link to the relevant documentation.

Something like this? I can create a PR if that helps!

1. Deploy (or run) your agent docs
2. Create a GitHub App docs
3. Configure App for Agent docs

[gr2m]

Pull request are always welcome, and improvements to documentation are best ✨

There is public documentation now (published yesterday): https://docs.github.com/en/copilot/building-copilot-extensions
Maybe we can reference some of it instead of recreating the same content, where applicable?

Thank you for your interest and your offer to help out!