docs: Add a LaTeX equivalent of the GameCube DSP User's Manual

The existing manual is nice to read and get up to speed on things,
however it has one major problem. It's actually a pain to extend or
modify, despite being released under the GNU documentation license: It's
in PDF format.

I've taken the time to reproduce what the document more-or-less looks
like in order to allow extending the document using open-source tools
while also using a more general solution like LaTeX, which can generate
the document into multiple formats if desirable.

This should make extending the document much easier (and not lock the
information into one concrete format).