Docs experience for first-time users

The current experience of docs.gitlab.com is a large list of links. For users new to Git and/or new to GitLab, this may be overwhelming, and unclear where to start. Can we give better guidance, or a good 'starting place' for new users?

Proposal

• Add intro to what GitLab is and basic structure of docs to main CE readme, and then links to 3 main sections with graphics
• Graphic for User guide section
• Graphic for Administration guide section
• Graphic for Developer guide section
• Create User guide index, move links to this section
• Create Administration guide section, move links to this section
• Create Developer guide section, move links to this section