Add database connection planning

What does this MR do?

This merge request adds comprehensive documentation for planning database connections in GitLab installations. The changes create a new PostgreSQL tuning guide that explains how to calculate the number of database connections needed based on your GitLab configuration (like number of Puma workers, Sidekiq processes, etc.).

The documentation includes formulas and examples for both single-node and multi-node setups, with special consideration for GitLab 16.0+ which uses dual database connections and effectively doubles connection usage. It also adds cross-references from the Puma and Sidekiq configuration pages to this new tuning guide, and updates the installation requirements to specify a minimum of 400 database connections with guidance to calculate the actual needs.

The goal is to help administrators properly size their PostgreSQL max_connections setting to prevent connection exhaustion and performance issues.

Related issues

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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• If you're adding a new page, add the product availability details under the H1 topic title.
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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

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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