Stable6 apps documentation

This is the first time I work on documentation. Initially I wanted to write some documentation for search lucene, but I first wanted to understand and then clean up what I found. As a starter @ser72 added a lot of documentation with images etc for various apps. This first commit of tis PR moves app specific dosumentation to an apps folder. Then, I clean up some of the markup and remove duplicate image references. Finally, I add the new documentation to the `index.rst` and `content.rst` so it becomes navigatable. Since I am new to the documentation repo and this is a huge change I am submitting this as a PR for @danimo and @DeepDiver1975 to comment on. Could you educate me and @ser72 on how to add a new apps section to the documentation or point out someone who can?

known issues:
- [ ] @ser72: some images use screenshots from the enterprise theme
- [ ] there are more folders in the admin_manual that fit better into a subdirectoy like configuration. should be cleaned up in a separate PR.