doc: Generate keywords table automatically

doc: Generate keywords table automatically

The SQL keywords table in the documentation had until now been
generated by some ad hoc scripting outside the source tree once for
each major release.  This changes it to an automated process.

We have the PostgreSQL keywords available in a parseable format in
parser/kwlist.h.  For the relevant SQL standard versions, keep the
keyword lists in new text files.  A new script
generate-keywords-table.pl pulls it all together and produces a
DocBook table.

The final output in the documentation should be identical after this
change.

Discussion: https://www.postgresql.org/message-id/flat/07daeadd-8c82-0d95-5e19-e350502cb749%402ndquadrant.com