Add admin commands docs, static musl binary, and comprehensive README improvements

[vadv]

Summary

Commit 1: Admin commands & static binary

• Document PAUSE / RESUME / RECONNECT admin commands in README: comparison table row + dedicated section with SQL examples
• Add static-binary job to release workflow — builds fully statically linked binaries via rust:1.87.0-alpine (musl), uploads pg_doorman-linux-amd64-static and patroni_proxy-linux-amd64-static as release artifacts

Commit 2: Comprehensive README improvements

Add 7 new documentation sections based on competitor analysis (PgBouncer, Odyssey, PgPool-II):

• Pooling Modes — transaction vs session mode explanation
• SQL Feature Compatibility — matrix of what works in each pooling mode (inspired by PgBouncer features page)
• TLS / SSL — client-facing and server-facing TLS configuration with mode reference table
• Monitoring — Prometheus metrics endpoint configuration and key metrics table
• Signals & Zero-Downtime Upgrade — signal reference table, binary upgrade procedure, systemd integration
• Docker — container usage example with environment variables table
• Coming from PgBouncer? — migration guide with INI→YAML parameter mapping table

Commit 3: Advisory locks & TLS security

• Fix advisory locks documentation: session-level pg_advisory_lock is unreliable with any pooler, recommend pg_advisory_xact_lock instead
• Add TLS Security defaults subsection: TLS 1.2 minimum, Mozilla Intermediate ciphers, full SAN+CN hostname verification
• Add comparison table row highlighting Odyssey's TLS 1.0 and weak cipher defaults

README grows from 201 to 375 lines (15 sections total).

Test plan

• Verify README renders correctly on GitHub (all tables, code blocks, links)
• Trigger release workflow on a test tag and confirm static binaries appear in artifacts
• Verify static binaries are truly statically linked (file ... | grep "statically linked")

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