[5.1.x] Fixed #36227 -- Fixed outdated PostgreSQL documentation links.

Backport of 3ecaa85 from main.

Author: Hesham942

docs/ref/contrib/postgres/indexes.txt

@@ -62,7 +62,7 @@ available from the ``django.contrib.postgres.indexes`` module.
         The ``deduplicate_items`` parameter was added.
 
     .. _fillfactor: https://www.postgresql.org/docs/current/sql-createindex.html#SQL-CREATEINDEX-STORAGE-PARAMETERS
-    .. _deduplicate_items: https://www.postgresql.org/docs/current/btree-implementation.html#BTREE-DEDUPLICATION
+    .. _deduplicate_items: https://www.postgresql.org/docs/current/btree.html#BTREE-DEDUPLICATION
 
 ``GinIndex``
 ============
@@ -72,7 +72,7 @@ available from the ``django.contrib.postgres.indexes`` module.
     Creates a `gin index <https://www.postgresql.org/docs/current/gin.html>`_.
 
     To use this index on data types not in the `built-in operator classes
-    <https://www.postgresql.org/docs/current/gin-builtin-opclasses.html>`_,
+    <https://www.postgresql.org/docs/current/gin.html#GIN-BUILTIN-OPCLASSES>`_,
     you need to activate the `btree_gin extension
     <https://www.postgresql.org/docs/current/btree-gin.html>`_ on
     PostgreSQL. You can install it using the
@@ -86,7 +86,7 @@ available from the ``django.contrib.postgres.indexes`` module.
     parameter to tune the maximum size of the GIN pending list which is used
     when ``fastupdate`` is enabled.
 
-    .. _GIN Fast Update Technique: https://www.postgresql.org/docs/current/gin-implementation.html#GIN-FAST-UPDATE
+    .. _GIN Fast Update Technique: https://www.postgresql.org/docs/current/gin.html#GIN-FAST-UPDATE
     .. _gin_pending_list_limit: https://www.postgresql.org/docs/current/runtime-config-client.html#GUC-GIN-PENDING-LIST-LIMIT
 
 ``GistIndex``
@@ -103,7 +103,7 @@ available from the ``django.contrib.postgres.indexes`` module.
     fields <range-fields>`.
 
     To use this index on data types not in the built-in `gist operator classes
-    <https://www.postgresql.org/docs/current/gist-builtin-opclasses.html>`_,
+    <https://www.postgresql.org/docs/current/gist.html#GIST-BUILTIN-OPCLASSES>`_,
     you need to activate the `btree_gist extension
     <https://www.postgresql.org/docs/current/btree-gist.html>`_ on PostgreSQL.
     You can install it using the
@@ -116,7 +116,7 @@ available from the ``django.contrib.postgres.indexes`` module.
     Provide an integer value from 10 to 100 to the fillfactor_ parameter to
     tune how packed the index pages will be. PostgreSQL's default is 90.
 
-    .. _buffering build: https://www.postgresql.org/docs/current/gist-implementation.html#GIST-BUFFERING-BUILD
+    .. _buffering build: https://www.postgresql.org/docs/current/gist.html#GIST-BUFFERING-BUILD
     .. _fillfactor: https://www.postgresql.org/docs/current/sql-createindex.html#SQL-CREATEINDEX-STORAGE-PARAMETERS
 
 ``HashIndex``