This updates the definitions and examples for some of the metadata,

making it more clear how some are used and what values are expected.
Some of the required keys are now marked as recommended instead, since

it doesn't always make sense to include them. "Required" show now be

used sparingly.
The file stats metadata key is also now recommended, instead of merely

optional, as it's a very useful thing to include if available for

statistics information.

Built the docs. Read through the new changes. Checked the links for

errors.