Improve Crowd DC installation doc

The current section Set up the shared directory could be more detailed to avoid confusion. Misinterpreting this section can lead to setting up the shared folder incorrectly, which causes issues like this:

• https://confluence.atlassian.com/display/CROWDKB/Logging+into+Crowd+results+in+a+blank+screen

This is a tricky problem to detect as there are no clear indications that those errors can interfere with the log-in process. Maybe the doc could be more detailed on the steps and bring a reference of what should and should not be shared between nodes like in the KB.