Document all filesystem directories k0s uses

[jnummelin]

We need to document the directories k0s puts data on. Also/especially from the point of view which ones are affected by --data-dir option.

There's some confusions on where k0s puts which things, for example #3508 and #1842

For each dir k0s uses, we should document at least:

• what's here
• why here
• does --data-dir affect this

[twz123]

When documenting this, it should be clear which folders are intended to be used by k0s users, i.e., which of them are subject to the semver guarantees, and which of them are considered internal and may change without notice.