meson: add docs, docs_pdf options

meson: add docs, docs_pdf options

Detect and report if the tools necessary to build documentation are available
during configure. This is represented as two new options 'docs' and
'docs_pdf', both defaulting to 'auto'.

This should also fix a meson error about the installdocs target, when none of
the doc tools are found.

Reviewed-by: Justin Pryzby <[email protected]>
Discussion: https://postgr.es/m/20230325201414[email protected]
Discussion: https://postgr.es/m/ZB8331v5IhUA/[email protected]