Add contrib function references in the doc index

doc/src/sgml/dblink.sgml

doc/src/sgml/earthdistance.sgml

doc/src/sgml/fuzzystrmatch.sgml

doc/src/sgml/hstore.sgml

doc/src/sgml/intagg.sgml

doc/src/sgml/intarray.sgml

doc/src/sgml/isn.sgml

doc/src/sgml/ltree.sgml

doc/src/sgml/pageinspect.sgml

doc/src/sgml/pgbuffercache.sgml

doc/src/sgml/pgcrypto.sgml

doc/src/sgml/pgfreespacemap.sgml

doc/src/sgml/pgrowlocks.sgml

doc/src/sgml/pgstatstatements.sgml

doc/src/sgml/pgstattuple.sgml

doc/src/sgml/pgtrgm.sgml

doc/src/sgml/sslinfo.sgml

doc/src/sgml/tablefunc.sgml

doc/src/sgml/uuid-ossp.sgml

doc/src/sgml/xml2.sgml