Restructure plugin documentation

[ang-zeyu]

Is your request related to a problem?

There's been quite a few developer side plugin apis added to the using plugins page, which pertain more to writing plugins than using them.
Since plantUML is also a plugin, but is the top site nav layer, we could restructure plugin documentation to be like so for better organization:

Describe the solution you'd like

- userGuide.md
  - ...
  - authoring contents
  - Using plugins
    - plantUML plugin
    - default plugins ( anchors, etc )
    - algolia
    - ...
- devGuide.md
  - Developer's guide
    - Writing plugins

Describe alternatives you've considered

na

Additional context

na