Support micrositeExtraMdFiles with mdoc

[seldridge]

I noticed that when switching from tut to mdoc, that the micrositeExtraMdFiles option doesn't seem to be supported. This manifests as the requested additional markdown not being copied and not showing up in the built site.

Taking a quick look at how the extra markdown is built using tut it appears that this simply isn't being done for the mdoc backend. It would be great if this was in there and I think it would be pretty straightforward to either replicate the tut logic for mdoc or generalize both.

(No promises...) but if I have some free personal time, I can see about a PR. Anyway, thanks for a great project!

[rafaparadela]

I'll take a look at this. Thanks for reporting @seldridge