Move Gitaly configuration documentation to CE

I'd like to move the configuration documentation of Gitaly to CE. Right now, we have https://docs.gitlab.com/ee//administration/gitaly/ which links to https://gitlab.com/gitlab-org/gitaly/blob/master/doc/configuration/README.md and vice versa.

Since there's only one doc for Gitaly, that would be easy to just copy its contents in the CE doc and have a single source of truth.

Todos

• Copy content to https://docs.gitlab.com/ee/administration/gitaly
• Link to it from https://gitlab.com/gitlab-org/gitaly/blob/master/doc/configuration/README.md explaining the new location
• Add reference to https://gitlab.com/gitlab-org/gitaly/blob/master/config.toml.example
• Copy content of gitaly-debug https://gitlab.com/gitlab-org/gitaly/blob/master/cmd/gitaly-debug/README.md (under troubleshooting section https://docs.gitlab.com/ee/administration/gitaly/#troubleshooting-gitaly-in-production)
• Update main README with the information and maybe copy the architecture diagram to the docs