Update documentation for 0.3

Update documentation means updating descriptions and screenshots to match 0.3 version (to be released).
As almost everything changed from 0.2, all the documentation has to be reviewed.
Also, "blurred" screenshots should be replaced by normal screenshots.

Will certainly take xwiki-devs and xwiki-users mailing-lists as examples, with a set of pre-defined types (like annoucements, brainstorming, ideas, contribs, ...).