[juw3x]

Added Spring Boot Connection Pool Exhausted error documentation.

This is a common production issue where HikariCP connection pool exhausts all available database connections, causing request timeouts and service unavailability.

The page includes:

• Verbatim error messages
• Root cause analysis
• Diagnostic commands for Spring Boot actuator and PostgreSQL
• Resolution steps including pool sizing and leak detection
• Prevention strategies

PR checklist:

• Created the page with correct structure
• All 14 sections filled in
• Error Message is verbatim
• Diagnostic Commands are read-only
• Front matter complete with valid severity, slug, tags, related, last_reviewed
• Updated errors/README.md with new category
• Created errors/spring-boot/README.md