Make an "Elements" listing page

Migrated issue

Reported by: jhodgdon

Related to replacing the Form API reference, it would be good to have an "Elements" page as one of the listing pages (along Functions, Classes, Services, etc.). See #1617948: [policy for now] New standard for documenting form/render elements and properties.

This page would locate all classes with FormElement or RenderElement annotation, and parse out the machine name of the element. Then it would make a list something like:

Element	Type	Class	Description
button	FormElement	Button [would link to class page]	(the description from the first line of the class)

To do this, the parser would need to extract the element machine names and save them as References. It's already doing references for Annotation, so this would be just a bit added to that code, because the element name is part of that annotation line.

Then the table would be generated from Views like the other listing pages.

Related issue: Issue #1617948