Implement MessageCommand in htmx

Problem/Motivation

Document how to implement MessageCommand using htmx

Proposed resolution

Document at https://www.drupal.org/docs/develop/drupal-apis/htmx/ajax-api-to-htmx/messagecommand

Remaining tasks

Edit linked page and add documentation

User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet

None