There should be a proper user manual for Riot

**Is your suggestion related to a problem? Please describe.**
As the owner of an installation of Synapse+Riot Web, and having had users install Riot clients for mobile devices, I should be able to supply them with an up-to-date user manual geared towards regular (i.e. non "power") users.

**Describe the solution you'd like**
A user manual goes in depth into all the menus, options and screens of the app. It helpfully breaks down all the functionality of the app so that users get a sense of all that can be accomplished with it.

It also walks them through procedures (e.g. cross-signing, key backup, SSO, signup, mutual user verification...).

The user manual is updated and released at the same time as any stable release of the app.

It is located in an easy to find location (e.g. following a "User Manual" link on the apps that takes people to the official riot website, or the website of the Riot hosting provider).

**Nice to have:** it exists in different formats (online and offline html, pdf...)

**Describe alternatives you've considered**
N/A

**Additional context**
The switch on cross-signing has dramatically put the emphasis on the absence of a proper user manual that would walk regular users of Riot through the meaning and procedures of setting up encryption.

I currently have to support a number of users who misunderstand the terminology, and generally don't grasp the concept of keys and get frustrated with Riot. Most of them separately use apps like WhatsApp or Signal but can't derive experience from using these to Riot.

I was helpfully pointed to https://about.riot.im/help but at this point, I think Riot (in all its flavours) has long outgrown the scope of a FAQ being sufficient.

The issue names no repository files, tests, or entry point. Start by reviewing the existing FAQ at https://about.riot.im/help and the requested coverage for menus, encryption, cross-signing, key backup, SSO, signup, and verification. Before implementation, agree on the manual's scope, location, formats, and release-update process; completion is not otherwise defined.

Written by the indexing model from the issue text.