Replicate installation guide for Operator on docs page

Currently we have all documentation for the Operator with the repo.

Can we move the installation steps to be on this page, and formatted in a similar fashion to the chart installation guide.

Relevant merge requests:

• Update the docs gitlab-org/charts/gitlab!1995 (merged)
• Redirect old deprecated page gitlab-org/gitlab!70908 (merged)
• Add to docs navbar gitlab-org/gitlab-docs!2161 (merged)