docs: how-to guide

This MR covers the instructions from https://git.glasklar.is/system-transparency/core/system-transparency/-/blob/main/doc/guide.md excluding the remote attestation stuff, yet.

My approach is to write good how-to guides and create explanation- and reference pages as needed on the go, when I feel it could be liked in the how-to. Trying to stick very close to https://diataxis.fr/how-to-guides/

Still struggling sometimes when to put stuff where (reference vs. explanation) and where to draw the lines between two different pages. Aiming for not ending up with describing everything at once, but also avoiding spreading the information across to many different files.