Scope of this bug is to track discussion and implementation of a maintainable Messaging System documentation on experimenter https://experimenter.info/messaging/messaging-surfaces/ that doesn't run out of date.

Audience of this documentation:

1. Product
2. QA
3. UX
4. DS
5. Messaging system customers

Purpose of this documentation:

1. Easily maintainable
2. Highlights available messaging surface
3. FAQs of messaging system customers

Existing documents created and maintained by audiences
User engagement surfaces DS perspective - https://mana.mozilla.org/wiki/display/DATA/User+Engagement+Surfaces

Messaging surfaces UX perspective - https://mana.mozilla.org/wiki/display/FIREFOX/Repository+of+current+UX
https://projects.invisionapp.com/share/JEQW8IBZHUV#/screens/353126959
https://miro.com/app/board/o9J_ljMvwmg=/

Message collision - infrastructure work - product perspective:
https://docs.google.com/document/d/1NeSGYe8xPx7J6t4OWnlx2Qa7B__7iIMMSor1yeNoCAg/edit

Messaging system - Engineering perspective
https://firefox-source-docs.mozilla.org/toolkit/components/messaging-system/docs/index.html
https://docs.google.com/document/d/1wEQEXjcSZGE8VP5qLmfS9MRmbbLY02R_4Z8-TL8OG_I/edit#
https://docs.google.com/document/d/1o8W-gEkgw2GC7KFSfQRkKfWzNJg1-6fpkVPrYmmot4Y/edit

Messaging System - Content perspective
https://docs.google.com/document/d/1nhkYbNUxguj1u_NnZl_D-Hq9uOw5obrIFewUvFnckDo/edit#heading=h.rjjti5jbcg61

First PR to experimenter docs: https://github.com/mozilla/experimenter-docs/pull/174

The PR above was accepted and Messaging System is now a part of Experimenter Docs.

Second PR to add available targeting link: https://github.com/mozilla/experimenter-docs/pull/177

We may want to include sections from this draft overview of the messaging system that are missing in our section the Experimenter docs, like Remote Localization and Message Groups.

Opened https://github.com/mozilla/experimenter-docs/issues/370 to update telemetry documentation in experimenter.info

Other than above telemetry source, DS uses below sources for analysis

The Glean Dictionary - https://dictionary.telemetry.mozilla.org/
The Probe Dictionary - https://probes.telemetry.mozilla.org/
GLAM - https://glam.telemetry.mozilla.org/?currentPage=1