[bug#40815] gnu: Add metamath

[B. Wilson]

This is my first packaging attempt, so careful critiques are very welcome.

The package definition itself is pretty bog standard, apart from how the "doc" 
output is supplied. Upstream provides the official documentation as a pdf 
offered separately from the source. I decided to include this as an input and 
manually copy it over. Upstream does also have a repo with the TeX sources. 
Would it be better to typset it directly instead?

Also, regarding my `install-doc' phase, is the way I copy over the 
/gnu/store/<hash>-metamath.pdf file reasonable? Unfortunately, `install-file' 
doesn't allow renaming the destination, so I had to mimic its effect. Is there 
a better, or more idiomatic way to do this kind of thing?

Anyway, cheers and guix!

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