TOPIC:

Usually, the web server is not switched off at night, I think

Pffff.... I guess you are the one that secretly check every night..? :-P

No, seriously: There can always be a disruption in the service, so the site may be down, while someone still wants to install the software.
But let's be fair:

For installation matters, a simple readmy.txt would do. For documentation matters...: An online solution. You've convinced me there

So, for an online solution, the decision would have to be between a (community-authored) wiki and a set of (team-authored) documents.
In all fairness: I'm just not a fan of a wiki, as it mostly ends up messy. Just take another look here:
www.kunena.org/docs/How_to_delete_a_user

A simple article with 2 big fat warnings on top... more warnings then the actual text. And also (Diesl): Note that everybody forgot to write a version sticker in this article. So..: Which version does this apply to?

All in all, I'm in favour of team-authored articles for documentation.

Of course, some well-written articles are great. But you have to write them first, or find some people writing them

A wiki-based solution can help to attract more people to write something. A team could help to tidy up pages?

My suggestion would be to use a wiki too,.