Doc Bug: Add information about Core Plugin support

[hamishwillee]

As per discussion here we should add information about core plugin support.

• The plugin API
• What you can and cannot do with core plugins.

E.g. in that issue "...allows a core plugin to override the default setup for the Values Widget."

[hamishwillee]

@DonLakeFlyer How do you want to approach this? I can see you may be way too busy to address this now, but is it worth adding some notes here in bullet form?

• what the API is for (ie what is a core plugin and who would use it)
• where the plugin API is defined
• some example implementations that have been done (link to github) showing how they are used.
• Integrating with other parts of the system.

[DonLakeFlyer]

I'll try to get a high level explanation in with details later.

[hamishwillee]

Works for me. I like to create lots of issue reports with as much detail as possible so we can backfill later.