Update docs/Running-a-Mirror

This MR updates the docs based on my experience working through them when setting up my mirror.

• Update disk usage from 2019 to the more realistic 2024 value.
• Add "privacy policy" and "test your mirror" to the instruction steps (privacy policy shouldn't be optional)
• Remove /health. As far as I see monitoring is now done based on the index.xml (which makes much more sense)