WIP: Add `services` to docs

What does this MR do?

This document describes a standard architecture of adding services to GitLab.

It documents the usage of health-checks, metrics, signals, lifecycle

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Style guides

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team