More detailed installation instructions

This was my first time with Shinken and I got tripped by the outdated docs on the .org site. Had trouble finding the readthedocs doc (probably because of google indexing) but still the doc there wasn't detailed for a first time module installation (I guess I wouldn't be the only first time installer looking into webui as a first module), had to search in the forum, undesrstand there was some kind of video or tutorial, and finally found more detailed instructions on Naparuba's course's github. How about also updating the shinken.io webui module installation steps!