Add better scaling docs for Gitaly
Problem to solve
There are dedicated docs about scaling GitLab https://docs.gitlab.com/ee/administration/scaling/, but there aren't clear docs for scaling the Gitaly specifically.
Further details
Proposal
Add docs like administration/gitaly/scaling
to document different ways to scale Gitaly:
- storage by sharding
- resources by clustering (Gitaly HA)
We might also want to update https://docs.gitlab.com/ee/administration/repository_storage_paths.html