GitHub OAuth tutorial references removed session/cookie API documentation

[codevogel]

Issue Summary:
The GitHub OAuth tutorial is broken due to missing documentation references after recent changes to the Sessions section.

Details:
Commit 5dbdccc in #1801 significantly overhauled the Sessions section documentation. While this appears to align with making Lucia a more general authentication guide, it has created a documentation gap that breaks existing tutorials.

Problem:
The GitHub OAuth tutorial still references the old Sessions documentation:

"Before starting, make sure you've created the session and cookie API outlined in the Sessions page."

However, these specific session and cookie API creation instructions no longer exist in the updated Sessions section. This leaves users unable to complete the tutorial as the OAuth implementation depends on APIs that are no longer documented.

Expected Resolution:
Either:

1. Update the GitHub OAuth tutorial to reference the current Sessions documentation, or
2. Restore the missing session/cookie API documentation that tutorials depend on

Sidenote:
This kind of thing happening with lack of review to the other tutorials makes me hesitant to recommend any of the Lucia guides at the moment.
I was writing a course on web development which refers to Lucia for the authentication section.
Now that the core documentation was restructured this heavily without any real heads up (at least I don't think this was visible on the lucia-auth site itself?), it's hard to trust that these tutorials today won't be referencing dead links tomorrow. It would be nice to have some kind of notice when major docs are getting overhauled so my students don't get stuck halfway through implementing something.