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 <[email protected]>
Discussion: https://postgr.es/m/20230209203855[email protected]
Discussion: https://postgr.es/m/20231103163848[email protected]