Introdcue a `FormatInfo` class that, very much like `ModuleInfo`
can be used to obtain meta information about a format. Methods
are added to `Index` to allow the enumeration of available formats,
getting the `FormatInfo` for a format given its name and to detect
a format via the manifest description data.
Change the top-level documentation to reflect the changes. Also
remove an outdated section about validation of the schema; this
was moved to the format specific code some time ago.