Add a generic 'note' container

[vadi2]

Similar to how note-to-balloters and stu-note exist, it would be really helpful to have a generic 'note' container. An example where this could be used is CDS Hooks' frontpage note:

[vadi2]

There also seems to be desire for a 'note to implementers' (fhir chat). I can file this one separately if desired.

[lmckenzi]

We don't see a strong reason for both 'note' and 'note to implementers' so we'll go with the simpler 'note' structure. We'll allow a contained "note-title" to provide a heading for the note. Will follow the same style approach as our other notes, with a slightly different color (maybe grey?)