Consolidate HA pages in docs and marketing sites

Problem

We have 3 pages talking about HA (and availability in general):

• https://docs.gitlab.com/ee/administration/high_availability/ - Old page that now has links to new docs and the marketing site.
• https://docs.gitlab.com/ee/administration/availability/ - New page that now has only a table.
• https://about.gitlab.com/solutions/high-availability/ - Marketing page with info that reads like docs.

We need to have only one HA page

Concerns

1. SEO: The marketing page ranks high in Google searches, so we can't just remove it.
2. SSoT: While we should strive for a single source of truth (the docs site of course 😛 ), we cannot remove either pages. People still search the docs and we need to have a page that talks about HA. This is also a known industry term that we should have in the docs search index.
3. We don't want to point users to those pages back and forth.

Proposal

1. !30781 (merged) Create new reference architectures page combining scaling and availability
2. gitlab-com/www-gitlab-com!48244 (merged) Change solutions to a “landing” page and link to new documentation for technical details.