pgsql: docs: meson: Change what 'docs' target builds

docs: meson: Change what 'docs' target builds

This undoes the change in what the 'docs' target builds 969509c3f2e. Tom was
concerned with having a target to just build the html docs, which a prior
commit now provided explicitly.

A subsequent commit will overhaul the documentation for the documentation
targets.

While at it, move all target in doc/src/sgml/Makefile up to just after the
default "html" target, and add a comment explaining "all" is *not* the default
target.

Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Discussion: https://postgr.es/m/20230209203855.njrepiupc3rmehfw@awork3.anarazel.de
Discussion: https://postgr.es/m/20231103163848.26egkh5qdgw3vmil@awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9e5b2a091fd0d672b7b2ff8afcfc9e9a6e9a5ac8

Modified Files
--------------
doc/src/sgml/Makefile | 4 ++--
doc/src/sgml/docguide.sgml | 2 +-
doc/src/sgml/meson.build | 4 ++--
3 files changed, 5 insertions(+), 5 deletions(-)