docs - updates for PostGIS 2.5.4 (#9989)
docs - updates for PostGIS 2.5.4 (#9989)

* docs - updates for PostGIS 2.5.4

-Updated version 2.1.5 -> 2.5.4
-Updated references to PostGIS topics
-Removed geometry and geography limitation
-Added enhancement for postgis_manager.sh script - support installing PostGIS in a non-default schema
-Added examples of updating postgis.enable_outdb_rasters and postgis.gdal_enabled_drivers for a GPDB session
-Remove ANALYZE limitation for user-defined data types
-Added  _postgis_index_extent function is not supported
-Added note: <-> operator returns centroid/centroid distance
-changed name of PostGIS aggregate function ST_MemCollect to ST_Collect

* docs - fix typo

* docs - minor changes to topic hierarchy.

* docs - one more minor hierarchy change.

* docs - update external URLs

* docs - fix typo, minor cleanup

* docs - found strange character
50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,jmcatamney/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb