Move HA Scaling information into a new HA/scaling page.

Why

We are currently trying to architecture our GitLab HA docs to be more clear and streamlined. Diana/Josh/Sid identified that on the HA home page we should move the scaling information and the reference architecture to a separate page.

https://docs.google.com/document/d/1XatsGYGepUx4l6Jpfx3b3XCKHn3vgJiI_A4wNNVp_mc/edit

What

• Move https://docs.gitlab.com/ee/administration/high_availability/#reference-architectures
• Move https://docs.gitlab.com/ee/administration/high_availability/#recommended-setups-based-on-number-of-users
• Move https://docs.gitlab.com/ee/administration/high_availability/#maturity-levels (later iteration for a re-write by Diana)