[ci-skip] Rails Guides: PostgreSQL timestamp with time zone

This guide addition documents how to configure your Rails application to
use the data type "timestamp with time zone" (timezone-aware timestamp),
instead of the default type, "timestamp without time zone"

Support for this data type was added earlier in Active Record, but
requires a configuration change in order to use, and from these PR
comments, where, why, and how to perform this configuration change
wasn't added to the Rails Guides Active Record PostgreSQL documentation
from what I could tell

Related PR: rails#41084

Guide page: https://guides.rubyonrails.org/active_record_postgresql.html

PostgreSQL best practices:
https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_timestamp_.28without_time_zone.29

Author: andyatkinson

guides/source/active_record_postgresql.md

@@ -516,6 +516,34 @@ irb> event.duration
 => 2 days
 ```
 
+### Timestamps
+
+* [Date/Time Types](https://www.postgresql.org/docs/current/datatype-datetime.html)
+
+Rails migrations with timestamps store the time a model was created or updated. By default and for legacy reasons, the columns use the `timestamp without time zone` data type.
+
+```ruby
+# db/migrate/20241220144913_create_devices.rb
+create_table :post, id: :uuid do |t|
+  t.datetime :published_at
+  # By default, Active Record will set the data type of this column to `timestamp without time zone`.
+end
+```
+
+While this works ok, [PostgreSQL best practices](https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_timestamp_.28without_time_zone.29) recommend that `timestamp with time zone` is used instead for timezone-aware timestamps.
+This must be configured before it can be used for new migrations.
+
+To configure `timestamp with time zone` as your new timestamp default data type, place the following configuration in the `config/application.rb` file.
+
+```ruby
+# config/application.rb
+ActiveSupport.on_load(:active_record_postgresqladapter) do
+  self.datetime_type = :timestamptz
+end
+```
+
+With that configuration in place, generate and apply new migrations, then verify their timestamps use the `timestamp with time zone` data type.
+
 UUID Primary Keys
 -----------------