Document attribute-based discovery in the handbook

Problem/Motivation

Follow-up from #3252386: Use PHP attributes instead of doctrine annotations and pointed out by @xjm. We should document attribute discovery in the handbook.

https://www.drupal.org/docs/drupal-apis/plugin-api has a section on annotations, could add a parallel one, and add note in the annotations page pointing to it.

There'll also be lots of individual examples of plugins using annotations that need updating over time to use attributes instead, but these will need to wait for individual subsystems to support attributes.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet