[Doc][Plugin] Improve visibility of Custom plugin docs

## Documentation Scope
This can improve the custom plugin visibility in main docs and community page

## Describe the Change
Please check screenshots below

## Screenshots
- The Custom plugin docs can be separated from plugins section for more visibility
- The Titile "Customize" can be renamed to "Implement a custom plugin"

- The above custom plugin implementation can additionally be linked to "Making Contributions"

## Additional context
There has been multiple instances where developers asked the question in the community on how to find the docs for testing custom plugins, finding resources etc.