Add list of supported font formats to `text-name` symbolizer doc

[samanpwbb]

Should add list of supported formats in the doc: https://github.com/mapnik/mapnik-reference/blob/master/3.0.0/reference.json#L1985

@springmeyer do you know this info off hand, or know of where to find it?

[springmeyer]

👍 you can grab the list from https://github.com/mapnik/mapnik/blob/master/src/font_engine_freetype.cpp#L67-L76, but honestly I would restrict to ttf, woff, otf, and ttc