reorganize sysadmin docs

I have made the sysadmin docs more granular. Instead of having a very long page with installation docs containing plenty of optional post-install configuration options, the installation docs now essentially only contain the docs on how to run the installer. The other parts were moved to sub-pages.

Besides this, I have added a new (more complete) page outlining system requirements for openBIS installation.

What remains to be done is the documentation of the openBIS docker image, which will be taken care of in the next update of the docs.