Docs: github-mode: Simplify `redirect_uri` instructions

[tordans]

This is a follow up to https://github.com/Thinkmill/keystatic/pull/1358 which makes the steps to get to the app easier.

I left the hints how to get to the list of all apps as well, but I think the setup process should always add the appSlug, even though this line is conditional.

Thanks for merging #1358!

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