Merge <docs/used-extensions> into <docs/requirements>
Update docs.

Merge <docs/modules> into <docs/comparation-with-yii>.
Update <docs/install-vkclone>.
Add note about author of theme in <docs/theme>.
Update database.php.example configuration file.
Some fixes in <docs/convetions> and <docs/index>.
Add xdiff installation guide in <docs/install-vkclone>.

Create <docs/install-xdiff>.
Update <docs/update-vkclone>.