Create an interface for column components other than MultiDocRef
We wanted to add some custom components (external links https://github.com/SciML/SciMLDocs/pull/240, commercial support icons, etc.) but the current structure of MultiDocumenter constrains this to MultiDocRef.
This PR introduces an abstract supertype of MultiDocRef, DropdownComponent (name up for review) where the only interface requirement is that the subtype implements render(::Component, doc, page, prettyurls). This can be used to create arbitrary components that are rendered as arbitrary HTML, within the column environment.
Looks like Windows tests are failing because of an extra carriage return (\r). Might add a chomp to that just to normalize whitespace.
We can make Link look pretty in a separate PR - this is ready from my end now!