Provide a non-technical guide to being a good client/server in the matrix ecosystem

Wha do you need to concentrate on maintaining as time goes on, and what are best practices for migrating and decommissioning a server or client. In a federated world we shouldn't just stop providing a client or server - we should consider the federation of other servers and clients as continuing afterwards.

Consider things like which sercrets are important / how to turn off a server neatly (leaving all rooms etc) / how to manage push notifications / what tools there are to stay in touch with your users before decommissioning a server / how to ensure that your users can keep access to encrypted messages / retaining secrets in case you need to restart the service.

Some of these things are "obvious" to long-term matrix developers or server admins, and while there's a chunk of resources on the technical side of things, there's not much on the meta side of things, and nearly none on how to neatly deprecate and remove services cleanly.

No files, tests, or entry points are named. Start by reviewing the existing technical resources mentioned in the issue and define the guide's scope around federation, migration, decommissioning, secrets, notifications, user communication, and encrypted-message access. Done means a clear non-technical guide explains how to maintain, migrate, and retire clients or servers safely.

Written by the indexing model from the issue text.