Expand Database Load Balancing docs

What does this MR do?

MR updates Database Load Balancing documentation:

• Updates the main Database Load Balancing page with up to date info and new config examples
  • Database Load Balancing is now recommended by default via host list (decision discussions here).
  • Move Database Load Balancing page to under PostgreSQL section. (related navbar MR)
• Updates the PostgreSQL and Reference Architecture docs in text and diagrams to reflect Database Load Balancing is now recommended

Related issues

Closes gitlab-org/quality/reference-architectures#52 (closed) #345133 (closed)

Relates https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/239

Author's checklist

• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.