Well, after a good bit of wrangling, here is another round.

We have URLs pointing to static sources; I got upstream to fix things so we
don't need a patch; and now we're generating the doc output's pdf from the TeX
source.

The latter is what made this take so much time and effort.

On the one hand, it required packaging up 6 texlive dependencies, which are
included in the attached patches. On the other hand, it turns out that the
texlive-amsfonts package is broken, which we need to typset the metamath doc
output. This caused me tons of grief but turns out to be a known issue:

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=40558

Thus, as of commit a1f84aca, the attached patch for metamath actually breaks.
However, with the proposed patched included in issue #40558 above, it builds
just fine.

Regarding the patches for the texlive packages, I did all of them as
simple-texlive-package templates with #:trivial? #t, grabbing the files from
tex/latex and doc/latex. Is this reasonable? Looking at other packages, I
cannot tell whether it'd be preferable to directly generate everything from the
*.dtx and *.sty sources.

Also, regarding texlive-mathstyle and texlive-flexisym, their files reside
under latex/breqn, but since they have their own ctan pages, I opted to split
them into separate packages and make the dependencies explicit. Does that seem
reasonable?

Anyway, thanks for taking a look! Hopefully, these look mergeable apart from
the texlive-amsfonts issue.