pgsql: doc: Move htmlhelp output to subdirectory

doc: Move htmlhelp output to subdirectory

This makes it behave more like the html output.  That will make some
subsequent changes across all output formats easier.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/477422c9d176784e90e630de3c92b4fae8590bd2

Modified Files
--------------
doc/src/sgml/Makefile          | 8 ++++++--
doc/src/sgml/stylesheet-hh.xsl | 2 ++
2 files changed, 8 insertions(+), 2 deletions(-)