Proposal to add a guideline to add a service in admin guide

What would you like to be added:

A guide note for how to add a service for pai.

Why is this needed:

When I add a docker-cache service by myself, there are many know-hows need to be explained.

1. service folder structure
   1. build(optional), config, deploy
   2. config folder include docker-cache.yaml and docker_cache.py (if service name is docker-cache)
      1. docker-cache.yaml will use as jinja template default parameters, {{ cluster_ cfg["docker-cache"]["<param_key>"] }}
      2. docker_cache.py is a preprocess script to add runtime parameter like pai-master ip
   3. deploy folder
      1. service.yaml describe template file need to be generate by jinja, and other meta info for service
      2. start.sh/stop.sh/refresh.sh/delete.sh wrapped k8s operation for service
      3. templates for k8s config
2. config folder yaml and py script naming convention: py need '_' replace '-'
3. deploy folder template {{ }} syntax and cluster-configuration structure: {{ }} to replace by jinja
4. deploy folder service.yaml structure
5. k8s yaml can't use namespace

Without this feature, how does the current module work:

Components that may involve changes:

Start by reviewing the existing admin guide and the service structure described in the issue, including config, deploy, service.yaml, templates, and the start.sh/stop.sh/refresh.sh/delete.sh scripts. Document the naming conventions, Jinja parameters, cluster-configuration structure, and namespace limitation; done means a newcomer can follow the guide to add a service without relying on undocumented know-how.