Simplify information architecture on Scaling documentation

What does this MR do?

This addresses the feedback from the last efficiency meeting. https://docs.google.com/document/d/10zaXBXkX31wfc0DUsRb5mzFswe7nxUNCthUggwGQvsc/edit#bookmark=id.lx1h75htq3rc

Sid: “Scaling” page: need to include what happens with 1-1k users. Single node or multi-node is a detail under Sid: Please change information architecture on scaling page H2: Reference Architectures

• Under 1k
• 1-2k
• Etc Whether single node or multi-node is an optional detail under each ref arch that does not need to be in the page index

Change made

• Removed notion of single-node and multi-node
• Added explicit heading for:
  • Under 1,000 users
  • 1,000 to 1,999 users
  • 2,000 users
  • 5,000 users
  • 10,000 users
  • 25,000 users
  • 50,000 users
• Consolidated generic information on GitLab architecture in the overview of H2: Reference Architectures instead.

Screenshots

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