docs: gitaly chef roles

What

• Show how the chef roles inherit from one another.
• Explain what each role is doing.

Why

Most people aren't sure where to put their configuration for Gitaly unless they have done so already, this helps us understand how our roles work in chef.

It also exposes the big problem we have with chef configuration where gstg and gprd configuration structure does not match.

Reference: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16744 Signed-off-by: Steve Azzopardi sazzopardi@gitlab.com