Split the spec into two

[Natalie Pendragon]

> This is a big overall improvement in readability and citeability.
> Thanks a lot for suggesting it.

This IS a big improvement!

A small additional idea - would you be open to hosting a `text/gemini`
version? I can definitely see the merit of the `text/plain` version,
for maximum ease of distribution, so maybe this is just asking for
trouble to host two versions, but I do feel at this point in Gemini,
most folks are probably consuming the Gemini spec in a Gemini browser,
within which readability would be substantially enhanced by even just
the addition of a bit of the usual client header line handling (would
you look at that, Gemini supports three levels of it, and now we only
*have* three levels of it in the spec :).