Separate scaling and high availability content

Purpose

Separates scaling and high availability content to better separate the two concerns.

Work is being done in !29002 (merged)

(Previous description, for historical purposes)

Why

In the meeting with Josh/Sid/Diana (notes here: https://docs.google.com/document/d/1XatsGYGepUx4l6Jpfx3b3XCKHn3vgJiI_A4wNNVp_mc/edit#) it was explained that we need to break down this page https://docs.gitlab.com/ee/administration/high_availability/ into more distinct sections, one of them being Availability.

What

A new page which covers:

• What availability means
• What considerations you should make when looking at GitLab architecture differences Josh already has an open issue here, which is probably the page that is needed to satisfy this issue
• ??