update install.md for ILIAS 11

[rfalkenstein]

First draft of the install documentation for ILIAS 11. Most parts were picked from release_10. On thing is not tested yet:

• What's the best way of doing a major upgrade from 10 -> 11. How to handle the folders "public", "Customizing" when doing an inplace-update as it is described in this documentation. Hint: Do we really want to document an inplace upgrade or should we better document how to create a new directory and change the targets for the necessary paths?
• We should pick this to trunk then and work from this on for ILIAS 12.