docs/doc-guide: Add documentation on SPHINX_IMGMATH

Now that building html docs with math expressions does not need texlive
packages, remove the note on the requirement in the "Sphinx Install"
section.

Instead, add sections of "Math Expressions in HTML" and "Choice of Math
Renderer".
Describe the effect of setting SPHINX_IMGMATH in the latter section.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/a67e3279-6bc7-ee2c-2b49-9275252460b0@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>