[DOCs] Add custom queries for MySQL (PMM-14502)

[catalinaadam]

Summary

• Adds a Running custom queries section to the MySQL connection guide, consistent with the existing PostgreSQL custom queries documentation
• Covers configuration path, resolution directories, YAML format, a practical example, and metric types
• Replaces the blog post as the primary reference (blog post link kept in Related topics)

What's documented

• Directory path: /usr/local/percona/pmm/collectors/custom-queries/mysql
• Three resolution tiers: high (5s), medium (10s), low (60s)
• YAML query definition format with a real-world InnoDB index stats example
• Metric types: GAUGE, COUNTER, LABEL, DURATION, DISCARD

Test plan

• Verify the custom queries path is correct for the current PMM version
• Confirm metric type list matches what mysqld_exporter actually supports
• Check the InnoDB index stats example query runs on a live MySQL instance

Fixes: PMM-14502