Follow-up from "Solutions High Availability page refinement"

Why is this change being made?

We want to keep the technical scaling and high availability page as the SSOT for technical treatment of the subject, and leave the solutions page as a more high-level discussion giving general capabilities. This MR moves the maturity model to the technical page and adds some detail about the simple scaling model.

See also gitlab-org/gitlab!27046 (merged)

/cc @ljlane @joshlambert @lyle @tcooney @sytses

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

Closes #6817 (closed)