warn about 'rmd' file extension in epsiodes folder

A point of confusion encountered in the Collaborative Lesson Development Training pilot: pages did not build from source files with the `.rmd` extension (the files had to be renamed to use `.Rmd` first). This is an easy mistake to make and I wonder if we can have a validation check that warns the user about this file extension and suggests the fix that they will probably want to make?

Start by locating the validation checks for source files in the episodes folder. Confirm how lowercase .rmd files are handled, then make the validation output warn about the extension and suggest renaming it to .Rmd; done means the warning appears for this case without affecting valid files.

Written by the indexing model from the issue text.