Add documentation for #12429

[RCheesley]

This adds the documentation for mautic/mautic#12429 (mostly copy/paste from the PR so please follow those instructions and test that it actually works!)

Note:

• We should pull the demo plugin into the Mautic org, I think, if we're referring to it in the docs. It should be something that we maintain and is subject to our governance processes. @Moongazer would you be OK with that?
• I've added links to the Plugins docs and the API - anything else that would be helpful for developers?

[RCheesley]

The build is failing here because I used a URL for the demo plugin bundle which is in the Mautic org, and it obvs isn't there yet :)