Copilot-cli: Add documentation explaining the scope of permissions for the GitHub access token

[efekarakus]

While running pipeline init we prompt users to enter their github access token so that we can pull their repository.
We should have instructions in our docs explaining how to do that and the minimal permissions necessary, and link to the docs from the CLI.

[kohidave]

We can probably provide a pre-filled out URL like this:

https://github.com/settings/tokens/new?description=AWS%20Copilot%20CodePipeline%20Integration&scopes=repo

(we'd need to tune the scope Param)

[efekarakus]

Since Copilot added support for CodeStar connections for pipelines, we don't need to create an access token: https://aws.github.io/copilot-cli/docs/manifest/pipeline/. Resolving.