Create new reference architectures page

What does this MR do?

Review app: http://docs-preview-ee-30781.178.62.207.141.nip.io/ee/administration/reference_architectures/index.html

1. A single page doc/administration/reference_architectures/index.md in the docs that:
   • Describes the base set up (<1000 users)
   • Adds the reference architectures of the scaling page
   • Describes (or links to) the actual procedure for setting up the reference architecture
2. Adds the following 4 availability complexities
   1. Automated Backups
   2. Traffic Load Balancer
   3. Automated Database Failover
   4. Instance level replication with GitLab Geo
3. Deprecate all the other pages which are confusing users (and us) and fix backlinks to them:
   • https://docs.gitlab.com/ee/administration/scaling/
   • https://docs.gitlab.com/ee/administration/availability/
   • https://docs.gitlab.com/ee/administration/high_availability/
   • https://docs.gitlab.com/ee/administration/high_availability/gitlab.html => This refers to the GitLab Rails app, we can't deprecate it, but we should change the title.
   • We should make an honest effort to find any more, because it keeps seeming like there is a new discovery every day
4. There are some stylistic things we need to change, which Sid describes in the video https://www.youtube.com/watch?v=wV3lzzXUogs
   • "don't say 'simple'"
   • Don't link to the list of all performance issues below each ref arch.
   • Don't say "high availability" everywhere
   • don't say "A 3k ref arch is being worked on", just add it when it exists
5. "Scaling" & "Availability" are still in the lefthand sidebar, even though they redirect gitlab-docs!841 (merged)
6. The GitLab rails app page still exists, but the breadcrumb is broken. Let's rename it. It also seems to have a bunch of peer pages (visible in the left nav) e.g. /ee/administration/high_availability/database.html tackle in a follow-up #214353 (closed)
7. Add a link in "essential documentation" on the docs overview page: https://docs.gitlab.com/ee/README.html#overview
8. Under "Up to 3,000 users" it says "Supported users (approximate): 2,000" I assume this is a typo
9. Get rid of the sections: "GitLab reference architectures" (the top of page), "Overview", "Trade-offs", "Reference architectures", "Availability complexity" and rewrite something simpler and put in a chart.
10. The "Configure GitLab to scale" needs to be per-reference architecture. It's very disembodied tackle in a follow-up #216448 (closed)

Follow-ups:

• Move reference architectures components to a new location
• Document how to configure GitLab to scale per reference architecture
• Find occurrences of HA in the docs and change them to the new ref architectures page
• Re-evaluate the four levels of availability complexity in the reference architecture docs
• Refactor the requirements docs in regards to the reference architectures

Related issues

Addresses part of #215161 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
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