Fix broken links in PostGIS documentation (#8637)

Co-authored-by: Gregory Power <31050507+gregorywaynepower@users.noreply.github.com>

docs/training_manual/spatial_databases/index.rst

@@ -11,15 +11,15 @@ into the database and make use of the geographic functions that PostGIS offers.
 While working through this section, you may want to keep a copy of the **PostGIS
 cheat sheet**  available
 from `Boston GIS user group <https://www.bostongis.com/postgis_quickguide.bqg>`_.
-Another useful resource is the `online <https://postgis.net/docs/>`_ PostGIS
+Another useful resource is the `online <https://postgis.net/documentation/>`_ PostGIS
 documentation.
 
 There are also some more extensive tutorials on PostGIS and Spatial Databases
-available from Boundless Geo:
+created by Boundless that are now hosted on the PostGIS's website:
 
-* `Introduction to PostGIS <http://workshops.boundlessgeo.com/postgis-intro/>`_
-* `Spatial Database Tips and Tricks
-  <http://workshops.boundlessgeo.com/postgis-spatialdbtips/>`_
+* `Introduction to PostGIS <https://postgis.net/workshops/postgis-intro/>`_
+* `PostGIS Database Tips and Tricks
+  <https://postgis.net/documentation/tips/>`_
 
 See also `PostGIS In Action <https://www.postgis.us>`_.