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

Author: MasaoFujii

doc/src/sgml/catalogs.sgml

@@ -10455,8 +10455,9 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
 
   <para>
    The <structfield>pid</structfield> column can be joined to the
-   <structfield>pid</structfield> column of the <link
-   linkend="pg-stat-activity-view"><structname>pg_stat_activity</structname></link>
+   <structfield>pid</structfield> column of the
+   <link linkend="monitoring-pg-stat-activity-view">
+   <structname>pg_stat_activity</structname></link>
    view to get more
    information on the session holding or awaiting each lock,
    for example

doc/src/sgml/config.sgml

@@ -3889,9 +3889,9 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
         The synchronous standbys will be those whose names appear
         in this list, and
         that are both currently connected and streaming data in real-time
-        (as shown by a state of <literal>streaming</literal> in the <link
-        linkend="pg-stat-replication-view"><literal>pg_stat_replication</literal></link>
-        view).
+        (as shown by a state of <literal>streaming</literal> in the
+        <link linkend="monitoring-pg-stat-replication-view">
+        <structname>pg_stat_replication</structname></link> view).
         Specifying more than one synchronous standby can allow for very high
         availability and protection against data loss.
        </para>
@@ -4244,8 +4244,8 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
        Specifies the minimum frequency for the WAL receiver
        process on the standby to send information about replication progress
        to the primary or upstream standby, where it can be seen using the
-       <link
-       linkend="pg-stat-replication-view"><literal>pg_stat_replication</literal></link>
+       <link linkend="monitoring-pg-stat-replication-view">
+       <structname>pg_stat_replication</structname></link>
        view.  The standby will report
        the last write-ahead log location it has written, the last position it
        has flushed to disk, and the last position it has applied.
@@ -6694,8 +6694,10 @@ local0.*    /var/log/postgresql
 
          <para>
           The backend type corresponds to the column
-          <structfield>backend_type</structfield> in the view <xref
-          linkend="pg-stat-activity-view"/>, but additional types can appear
+          <structfield>backend_type</structfield> in the view
+          <link linkend="monitoring-pg-stat-activity-view">
+          <structname>pg_stat_activity</structname></link>,
+          but additional types can appear
           in the log that don't show in that view.
          </para>
 
@@ -7191,7 +7193,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         platforms.  You can use the <xref linkend="pgtesttiming"/> tool to
         measure the overhead of timing on your system.
         I/O timing information is
-        displayed in <xref linkend="pg-stat-database-view"/>, in the output of
+        displayed in <link linkend="monitoring-pg-stat-database-view">
+        <structname>pg_stat_database</structname></link>, in the output of
         <xref linkend="sql-explain"/> when the <literal>BUFFERS</literal> option is
         used, and by <xref linkend="pgstatstatements"/>.  Only superusers can
         change this setting.

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.

doc/src/sgml/logical-replication.sgml

@@ -495,7 +495,8 @@
 
   <para>
    The monitoring information about subscription is visible in
-   <link linkend="pg-stat-subscription"><literal>pg_stat_subscription</literal></link>.
+   <link linkend="monitoring-pg-stat-subscription">
+   <structname>pg_stat_subscription</structname></link>.
    This view contains one row for every subscription worker.  A subscription
    can have zero or more active subscription workers depending on its state.
   </para>

doc/src/sgml/logicaldecoding.sgml

@@ -347,7 +347,8 @@ $ pg_recvlogical -d postgres --slot=test --drop-slot
    <para>
     The <link linkend="view-pg-replication-slots"><structname>pg_replication_slots</structname></link>
     view and the
-    <link linkend="pg-stat-replication-view"><structname>pg_stat_replication</structname></link>
+    <link linkend="monitoring-pg-stat-replication-view">
+    <structname>pg_stat_replication</structname></link>
     view provide information about the current state of replication slots and
     streaming replication connections respectively. These views apply to both physical and
     logical replication.