Write a "Where's my stuff?" text explaining categories and tags

[albertocottica]

This stuff appeared in several places, mostly on the Matrix chat (where it's all but irretrievable).

A structured version was written by @tanius here. Even there it's written with the development team, not the users, as an intended audience.

[albertocottica]

@tanius do you have an initial writeup stashed somewhere, one for internal use? I can volunteer to adapt it for general consumption.

[tanius]

I think category and tag structure on our new platform have settled now, so let's document them. We even have a user manual now, where this content would be added in a sub-section of section 2.

Assigning this task to @anuzement :)

[tanius]

Done by @anuzement now, see the user manual in section "2.3. Using categories and tags".