doc/housekeeping: Document housekeeping strategies used by Gitaly

Patrick Steinhardtrequested to merge
pks-housekeeping-document-strategies into master

What does this MR do and why?

Document the two different housekeeping strategies that Gitaly uses to optimize repositories.

Closes #365318 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Merge request reports