📘 Documentation (docs/platform-agnostic-auth.md): research and document generic and efficient auth implementation

[KemingHe]

Note

This is a docs only task!

• Current production Buckeye GPT has seconds load time for StackAuth
• Beyond load time issues, Stack Auth is also close coupled to the app, req complete rewrite
• Our Mantine-base prototype should adopt advanced modularization and info-hiding to be auth-agnostic
• Before coding, plan out and document all options and pros and cons for each in platform-agnostic-auth.md in the docs/ dir