Provide more details on permissions needed by scriv github-release

[ncoghlan]

With fine-grained access tokens available on GitHub, it would be helpful to be more explicit about the permissions needed by scriv github-release.

From reading the code, I believe it is just the "Contents: write" permissions needed to access the following endpoints:

• https://docs.github.com/en/rest/releases/releases?apiVersion=2022-11-28#create-a-release
• https://docs.github.com/en/rest/releases/releases?apiVersion=2022-11-28#update-a-release

I'm not sure how best to word that in the docs, though (otherwise this would have been a PR instead of a docs issue report)