Added update guide for updating to 7.5, and pointed installation and updates guides to new version.
Merge branch '7-5-update-doc' into 'master'

Updated documentation for 7.5 release

Documentation for 7.5 release has been updated. Changes include:

- Pointed Ruby version in guides to 2.1.5
- Pointed gitlab-shell version in guides to 2.2.0

See merge request !1259