Codifies the conventions the playbook and its documentation already follow:
language and Markdown style, placeholder values for examples, the structure
of component documentation pages, and the checklist for adding a new
component (including where a component should live: this playbook, an
external MASH role, or mash-playbook).

Fixes #3631

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>