Initially, DiffX was envisioned as having a YAML-style metadata format.

This was an attempt to keep the appearance of metadata similar to that

of several diff variants, such as Git-style diffs. However, none of

these variants had any sort of grammar defined for their format. They

were just entirely ad-hoc.
We couldn't depend on YAML itself, due to numerous parser

inconsistencies. A home-grown grammar was defined, but this would be yet

another custom structured data format, and this seemed to be the wrong

place to introduce this.
Instead, we're now using JSON as the metadata format. This has a couple

significant advantages:


JSON is widely available in nearly any language, and through command

   line tools, making it very easy to support.


No part of JSON output should conflict with the syntax of DiffX,

   unified diffs, or any known custom diff format.


This change updates the spec to make it clear that JSON is the official

format used for DiffX, and to update all the example diffs and metadata

to use it.

Built the docs. Checked all the example diffs and metadata examples to

ensure they had correct syntax and was rendering correctly.