Show libraries as a seperate top-level object

[wlach]

It might be desirable to show libraries (and the metrics within them) as a seperate top-level object (alongside applications) within the Glean Dictionary. This would obviate the need for markdown-generated documentation for android-components (mozilla-mobile/fenix#19411 (comment)). Need to think on this a bit, but filing an issue so we have a reference in case more use cases come up.