Document OpenShift compatibility on a separate page

Proposal

Provide a separate section or page under https://docs.gitlab.com/ee/install/requirements.html about GitLab OpenShift support

This page should link to get started with

• installing GitLab on OpenShift
• installing Runner on OpenShift
• deploying to and integrating with OpenShift
  • including a list of not supported features depending on the Runners being within OpenShift or not