Support external markdown file for publication request intro

[ElliotSilver]

https://chat.fhir.org/#narrow/channel/196008-ig-publishing-requirements/topic/History.20page.20preview.20as.20part.20of.20autobuild/with/523793104 noted that other elements, such as description support both inline and external markdown files. It would be nice if all markdown elements (such as intro) supported an external file option.

[lmckenzi]

Will find a way to do this, though this isn't done using Jekyll like the others, so there's more complexity here.