Add file cleanup details for the metrics server

[neddp]

This PR documents the new automatic metrics file cleanup feature that was added in cloudfoundry/bosh#2654.

What changed?

• Added new "Metrics File Cleanup" section documenting the automatic cleanup of stale metrics files in /var/vcap/store/director/metrics
• Reflowed the entire document to remove manual line breaks (80-character hard wrapping) and use modern Markdown formatting with natural single-line sentences

There is a placeholder for the bosh version which needs to be populated later.