pgsql: 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 <pryzby(at)telsasoft(dot)com>
Discussion: https://postgr.es/m/20230325201414.sh7c6xlut2fpunnv@awork3.anarazel.de
Discussion: https://postgr.es/m/ZB8331v5IhUA/pNu(at)telsasoft(dot)com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/ef07d351a1860a6e93c89cc7af3ae13366942bf0

Modified Files
--------------
doc/src/sgml/installation.sgml | 33 +++++++++++++++++++++++++++++++++
doc/src/sgml/meson.build | 23 +++++++----------------
meson.build | 37 +++++++++++++++++++++++++++++++++++++
meson_options.txt | 9 +++++++++
4 files changed, 86 insertions(+), 16 deletions(-)