Add ZSMKit Docs as an Appendix

[m00dawg]

The spec is over here: https://github.com/tildearrow/furnace/blob/9d4a900d81879a176e2b5256b742187625930f31/papers/zsm-format.md?plain=1#L35

Would be nice to mirror it as an appendix to x16-docs since it's essentially the standard for playback solution on the X16

[tomxp411]

I'm not sure about adding documentation for third party tools to the PRG. While ZSMKit is handy, it's not available in the ROM or even bundled on the SD card.

What we should have, and don't, is an appendix with links to compilers, assemblers, and media toolchains like ZSMKit, image file converters, and so on.