Add an ability to have 'intro' and 'notes' on Operations tab for a resource

[lmckenzi]

From https://jira.hl7.org/browse/FHIR-20620

It would be good to have the ability to put files with a specific name (say operations-intro.xml and/or operations-notes.xml) into a resource folder and have it automatically put the specified content into the operations tab for that page above or below the list of operations respectively. This would allow pointing to useful context or externally-defined operations that may be of interest.

It might be useful to have the same capability for other tabs (e.g. profiles tab, mappings tab, etc.). Extensions tab lives elsewhere and would be a bit more complicated as those are generated pages.