Provide reference .rtf files + fonts

[madebr]

While porting SDL_rtf to SDL3, it would have been useful to have reference .rtf files and fonts available in this repository.

If copyright prevents this, perhaps easy instruction about how to obtain these would be useful as well. But some googling around showed me there do exist fonts in the public domain.

[sezero]

There is at least this rtf file from SDL-1.2 tree, which should really be free: https://github.com/libsdl-org/SDL-1.2/blob/main/Xcode/SDL/pkg-support/resources/License.rtf

[sezero]

Also, linuxdoc-tools can output to rtf using sgml2rtf

[madebr]

The thing is showrtf expects the user to pass a few fonts: https://github.com/libsdl-org/SDL_rtf/blob/b42410aa17e9e2b4c73532273d93906a200bd654/showrtf.c#L167

Perhaps we need to add a font enumeration api to SDL_ttf instead (using Fontconfig on Linux)