Rearrange link processing model stuff

[domenic]

The link processing model (covering Link headers, early hint versions of Link headers, and <link> elements) has grown organically in the spec over time. #7866 makes a good first step at cleaning that up in terms of algorithms.

After that I think we should also consider doing some rearranging. Here is my proposal.

4.2.4 The link element
   Processing the media attribute
   Processing the type attribute
   Providing users with a means to follow...
   (existing stuff moved below)

4.6 Links
   Introduction
   Links created by a and area elements
   ...
   Hyperlink auditing
   (new) External resource links processing model
       Shared infrastructure
       link elements
       `Link` headers
       Early hints
   Link types

Related: #6450 about how it'd be ideal to get rid of the "Processing the media attribute" / "Processing the type attribute" sections.

[noamr]

#7984 also cleans up a small duplication