Write documentation for the entity plugin

In ✨ Support for block content entity Needs work a document that I wrote a year ago has been road-tested by @grimreaper with some great feedback.

Now that we have the Gitlab Templates "Pages" job, it might be a better idea to keep the documentation stored within the Scheduler repo, rather than maintaining the pages on the drupal.org site. So I will use this issue to experiment with MkDocs and see how it goes.

Here are the plugin pages being worked on in this MR
https://project.pages.drupalcode.org/scheduler/plugin/

I have merged the first MR, but the documentation is not totally finished yet.