Add missing error explanations to docs

[balazsorban44]

Currently, the Errors documentation page has missing/empty sections all over.

We should document when these errors occur and describe how to resolve them, if possible.

[balazsorban44]

@lluia @ndom91 for starters, I think we could add these pages in the navbar in the docs for discoverability:

https://next-auth.js.org/errors

https://next-auth.js.org/warnings

Currently, I think they are only reachable if an error/warning is shown in your terminal.

[ndom91]

Yeah exaclty, they are not discoverable in the page / search.

EDIT: #2100

[ndom91]

Closing this issue - missing errors / warnings were described (in next docs - https://next-auth-git-next-nextauthjs.vercel.app/errors) and the pages are also now in the sidebar / findable in search 🎉