Update docs structure and content, merge with master.

Update docs structure and content, merge with master.

-Bring the last updates from master (from commit 298c8f9ab0147159e70e47fb7f0766a5bc1b8b31).
-Fix background of some images (use white instead of none).
-Remove the 'doc' dir and move all its content to root folder, so that the whole documentation can be consulted from the root directory.
-Update the documentation index.
-Update docs layout references to match the new directory structure.