Add metadata and crosslinks to database pages

What does this MR do?

Begins to handle the items identified as problematic in #381813 (closed) for groupdatabase:

• claim doc/install/postgresql_extensions.md as belonging to groupdatabase
• administration area: on doc/administration/postgresql/index.md,
  • Add a H2 so I can group the existing 3 items together as H3s underneath it
  • Add crosslinks to the developer docs in doc/development/database/
  • Add a related-topics section that includes links to the other pages in the directory
• developer docs: on doc/development/database/index.md,
  • Add crosslinks to the 3 most important pages in the administration area, so the developers have a way of knowing the administration-area info exists

Related issues

• Closes Review Postgres docs in /doc/administration (#381813 - closed)
• Related to Follow-up from "Add metadata and crosslinks to ... (#389206) which is the follow-up from #381813 (closed) and this work. The work should be handled by another team.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's 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 you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• 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.
  • Ensure a release milestone is set.
  • 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 reviews above. Maintainer's review can occur before or after a technical writer review.