Document sensible block categories

Problem/Motivation

This is a follow up to #2085201: Use sensible block categories (rather than module-name categories) which was committed and re-opened for a documentation update. This issue is to do that documentation work,

Actually, we need a documentation page in the handbook for this, under UX best practices. People don't know to use these categories right now and so we're starting to regress them in issues like #2020399: Convert "Who's online" block to a View.

From #39 of the original issue

Proposed resolution

Update documentation

Remaining tasks

Update documentation
Review

User interface changes

API changes

Data model changes

Release notes snippet