Improve `Basics` section in the docs

Part of https://github.com/blitz-js/blitz/issues/3828

Go over each page and see how it can be improved:
- Is there a guide?
- Is the API covered?
- Make sure that the information is correct and up to date with the Blitz 2.0

Start with the pages in the docs' Basics section and compare each one with Blitz 2.0. Check whether each page has a guide, covers its API, and remains accurate; done means every Basics page has been reviewed and the needed documentation updates are identified or made.

Written by the indexing model from the issue text.