Doc updates about storage, persistent disks, statefulsets.

[seizethedave]

What this PR does

Add docs clarifying the kind of volume to use for compactor's data-dir, and add a note about why compactor should use a StatefulSet.

Which issue(s) this PR fixes or relates to

Fixes #3061

Checklist

• [ ] Tests updated.
• [x] Documentation added.
• [ ] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [ ] about-versioning.md updated with experimental features.

[CLAassistant]

All committers have signed the CLA.