Adding draft documentation version for pg_wait_sampling

Author: Liudmila Mantrova

doc/src/sgml/contrib.sgml

@@ -152,6 +152,7 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
  &pg-tsparser;
  &pgvariables;
  &pgvisibility;
+ &pg-wait-sampling;
  &plantuner; 
  &postgres-fdw;
  &rum;

doc/src/sgml/filelist.sgml

@@ -160,6 +160,7 @@
 <!ENTITY pg-tsparser          SYSTEM "pg_tsparser.sgml">
 <!ENTITY pgvariables     SYSTEM "pg_variables.sgml">
 <!ENTITY pgvisibility    SYSTEM "pgvisibility.sgml">
+<!ENTITY pg-wait-sampling    SYSTEM "pg-wait-sampling.sgml">
 <!ENTITY postgres-fdw    SYSTEM "postgres-fdw.sgml">
 <!ENTITY plantuner       SYSTEM "plantuner.sgml">
 <!ENTITY rum             SYSTEM "rum.sgml">