[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.

[d4x1]

I think this advice is reasonable.
cc @Startrekzky @klesh

[Startrekzky]

Hi @JoshuaPoddoku The customize doc actually describes how to use the customize plugin rather than implement the plugin, that's why we put it under the "Plugin" folder. To increase the discoverability of this doc, I added an entry in the domain layer model doc. https://devlake.apache.org/docs/DataModels/DevLakeDomainLayerSchema#how-to-customize-data-models

If you want to further increase the visibility of the doc, instead of moving the doc's path, can you think of other ways to achieve this? For example, add a doc to the "Configuration" folder with the name "How to add customize data schema?" and link it to the doc, etc.