Beginning of format work, reframing and restructuring format info before diving into details

[lisad]

With this organization,

• Schema authors can probably just read the table near the top of the format section, currently 8.2
• Validator implementors can read all of section 8
• Extension authors can read the new section probably? If it doesn't end up very long can reconsider

This also reframes format as an expected feature, which I believe schema authors do expect. More work will be required to close the loop on that.