This adds a new page that covers how to customize the UI using the

actions framework and action hooks. It walks through building a basic

action, customizing it for different needs, introducing JavaScript

support, custom rendering, and then how to use actions in the main

places in which they're supported.
This replaces the old action documentation, which was out of date with

the new changes in 8.0 and were constrained to the ActionHook page.
This will give us something that we can reference in the release notes,

give developers a solid starting point for writing actions, and give AI

agents enough information and links to assist others with that process.

Checked for spelling, link, and build errors.
Ran through Claude Code to check for any noticeable issues, including

inconsistency issues with the sample code or descriptions of attributes

and methods.