Document that `/signin`, `/auth`, and `/authenticate-codespace/*` are incompatible with codespaces

[jsoref]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

Something related to GitHub codespaces.

What part(s) of the article would you like to see updated?

Add some text explaining the process involved, which path patterns won't work and suggesting alternatives.

Additional information

• @tanmayeekamath wrote something promising to add docs: 400 error if app uses /signin endpoint community/community#5204 (comment) -- I can't find any sign of these docs.