Docs: improve GitLab Installation page

Follow-up from meeting with Axil and Josh.

Proposal

Improve the installation index page to make it more helpful to choose the installations method that best fits the user needs.

How

Install overview:

  • SaaS
    • Options
  • Self-hosted
    • Omnibus
    • Charts
    • Source

What else

  • Make the install methods documented in the about website to link to more complete instructions in the docs In &2789 we're talking about moving all install methods in the docs site.
  • Make a complete Kubernetes + GitLab doc listing all the options for using GL + K8s
  • Import the Charts docs directly from its repo, but maintain the navigation structure in the docs site

cc/ @mikelewis @axil @joshlambert Am I missing something? Please feel free to update this issue's description.

Edited by Achilleas Pipinellis