Prefer luatex for documentation

[Werner LEMBERG]

In https://gitlab.com/lilypond/lilypond/-/merge_requests/1714 I
suggest that we prefer luatex for building the documentation.  What do
people think?

The main advantage of using luatex is complete microtype support; this
was activated recently in `texinfo.tex`, and XeTeX doesn't support it
in its entirety, lacking font expansion.

The microtype feature yields (a) less underfull lines (i.e., less
lines with overly large inter-word spaces), (b) less hyphenation, and
(c) a better 'grayness' of the pages, thus increasing legibility.
While (c) is not a big issue with technical documentation, (a) has
quite an impact IMHO, and (b) is valuable since it is always a good
thing to avoid hyphenation with keywords and the like because there
might be misunderstandings whether the hyphen is part of the keyword
or due to the line break.

Note also that documentation formatted with luatex has slightly less
pages, but this is rather insignificant.

Attached you can find an example that demonstrates (a), taken from the
MR.

You can read more about microtype principles in the documentation of
the LaTeX 'microtype' package, of which `texinfo.tex` supports a small
subset of the most important features.

  https://ctan.math.illinois.edu/macros/latex/contrib/microtype/microtype.pdf


    Werner