Full scaling HA doc installation steps

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

Follow-on to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9174

Add installation instructions for Full Scaling in the same fashion as we have now for Basic Scaling

• Links for PostgreSQL and Redis can be re-used for steps 1 and 2
• Create a new page or section for Sidekiq on its own
• Add a new section to GitLab application nodes pages to indicate how to disable Sidekiq
• Add a new section to GitLab application nodes and/or Gitaly docs pages to indicate how to configure multiple data storage backends.