docs: adding database pooling

[ahstn]

Description

• Adds a concise overview of PostgreSQL connection poolers focused on feature sets, routing behavior, benchmark evidence, licensing, and maintenance activity.
• Distills the locally provided comparative reports into a shorter reviewer-friendly doc.
• Re-checks key claims against current upstream docs, release notes, and repo activity as of 2026-03-10.
• Replaces a dense comparison-table style with per-pooler sections to improve readability and make feature differences easier to scan.