Docs: Move "maturity" model from solutions to technical high availability docs

What does this MR do?

In order to keep technical documentation in one place, this MR moves the technical discussion of a "maturity model" for scaling and high availability from the solutions page into the technical page for scaling and high availability.

See also gitlab-com/www-gitlab-com!43381 (merged)

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

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #210248 (closed)