doc: Update the layout of "Viewing Statistics" section.

This commit updates the "Viewing Statistics" section more like
the existing catalogs chapter.

- Change its layout so that an introductory paragrap is put above
   the table for each statistics view. Previously the explanations
   were below the tables.

- Separate each view to different section and add index terms for them.

Author: Fujii Masao
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/6f8a482c-b3fa-4ed9-21c3-6d222a2cb87d@oss.nttdata.com

doc/src/sgml/high-availability.sgml

@@ -895,7 +895,8 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
     </para>
     <para>
      You can retrieve a list of WAL sender processes via the
-     <xref linkend="pg-stat-replication-view"/> view. Large differences between
+     <link linkend="monitoring-pg-stat-replication-view"><structname>
+     pg_stat_replication</structname></link> view. Large differences between
      <function>pg_current_wal_lsn</function> and the view's <literal>sent_lsn</literal> field
      might indicate that the master server is under heavy load, while
      differences between <literal>sent_lsn</literal> and
@@ -904,7 +905,8 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
     </para>
     <para>
      On a hot standby, the status of the WAL receiver process can be retrieved
-     via the <xref linkend="pg-stat-wal-receiver-view"/> view.  A large
+     via the <link linkend="monitoring-pg-stat-wal-receiver-view">
+     <structname>pg_stat_wal_receiver</structname></link> view.  A large
      difference between <function>pg_last_wal_replay_lsn</function> and the
      view's <literal>flushed_lsn</literal> indicates that WAL is being
      received faster than it can be replayed.