This improves the Pygments lexer for DiffX, fine-tuning the rules and

adding a second lexer specifically for DiffX metadata rules.
This new lexer, diffx-metadata, is used for each of our metadata

examples in the DiffX specification.
The main lexer has had some refinements. It's now able to leverage the

standard Diff lexer for diff sections (along with support for binary

file delta lines), and it's more strict about the section names and

levels it's allowed to highlight (helping spot problems in any examples

that we may write).
As the new lexer rules require content in content sections, one of the

earlier examples of valid headers had to be updated to avoid having two

content sections in a row. Ideally, the lexer would be fine with blank

entries like this, but it's easier said than done with the way lexers

work. So, we're limiting the examples to one empty (final) content

section.

Built the docs. Checked every code block to make sure it was highlighted

right.