create user install docs

[ThomasWaldmann]

• install via pip, apt, ...
• moin command based steps to get a wiki working (do not use quickinstall.py or "m", users do not have this)

[ThomasWaldmann]

in general, no docs should refer to "m" except the developer install section (see #909).

also, docs should not refer to virtualenvs. we can explain this at one place in the development install section, but it is not relevant for normal users/admins, who just do apt install moin (or so).