Cleanups and documentation fixes

Notable changes:

 * Better docs handling by rake task (visible in rake `-T`)
 * Added a few missing docs to PT crate
 * PT API is *experimental* now
 * Old demos (that are not yet PT-compatible) are renamed, pending either porting or removal
 * Feature guards are going away, removed them from hal, drivers and build system support
 * The above breaks sched, but it's broken anyways due to PT.
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