[Documentation:Developer] Add Trigger Function Documentation (#551)

### What does this PR do?
There is now documentation on how to create trigger functions and edit
existing functions.

### Images

![image](https://github.com/Submitty/submitty.github.io/assets/51209504/509d89f4-6606-42b1-89fd-611fab9100ad)

_docs/developer/development_instructions/trigger_functions.md

@@ -0,0 +1,88 @@
+---
+title: Trigger Functions
+category: Developer > Development Instructions
+---
+
+Trigger functions are a useful tool to keep data in sync. When a
+database table's column is modified, a trigger function can be
+set up to activate and modify the database as appropiate.
+
+We use PL/pgSQL for our trigger functions, which is a procedural
+programming language part of PostgreSQL, the language our database
+queries are conducted in.
+
+### Editing an Existing Trigger Function
+To edit an existing trigger function, simply edit the appropiate file inside the
+``migrations/migrator/triggers`` directory. When you next run ``submitty_install``
+or the migrator manually, the trigger functions on the database will be updated
+with the changes to the code.
+
+### Creating New Trigger Functions
+To create a new trigger function,
+
+1.  First, navigate to the ``migrations/migrator/triggers`` directory. Then choose
+    between the ``master`` and ``course`` folder. Choose the ``master`` folder if
+    your trigger function is activated by changes to the master database, and similarly,
+    choose the ``course`` if your trigger function is activated by changes to the course
+    database.
+
+2.  Create your file ``trigger_function_name.sql`` with these contents:
+
+    ```
+        --
+        -- Name: trigger_function_name(); Type: FUNCTION; Schema: public; Owner: -
+        --
+
+        CREATE OR REPLACE FUNCTION public.trigger_function_name() RETURNS trigger
+        LANGUAGE plpgsql
+        AS $$
+            BEGIN
+                (Body)
+            END;
+        $$;
+
+    ```
+    Replace the body with the trigger function. We recommend
+    [this tutorial series](https://www.postgresqltutorial.com/postgresql-triggers/introduction-postgresql-trigger/)
+    on how to write them. Check other trigger functions we have for an idea
+    on the syntax. One of the most important ideas to note is that OLD.column represents
+    the column's date before the edit and NEW.column represents the column's data after the
+    edit.
+
+3. Create a migration (see [here](/developer/development_instructions/migrations)
+    on how to write a migration). Somewhere in the migration, include the following:
+
+    ```
+        CREATE OR REPLACE FUNCTION public.trigger_function_name()
+            RETURNS trigger
+            LANGUAGE plpgsql
+            AS $$
+        BEGIN
+            RETURN NEW;
+        END;
+        $$;
+        
+        CREATE TRIGGER some_name_about_your_trigger_function
+        BEFORE/AFTER UPDATE/INSERT/DELETE ON public.the_table_your_function_is_activated_by
+        FOR EACH ROW EXECUTE PROCEDURE public.trigger_function_name();
+    ```
+    There's a few things to break down here. Your trigger function can be actived before or after
+    a table has a deletion, an insertion, or a value update on it. Pick either ``BEFORE`` or ``AFTER``. Pick either ``UPDATE``, ``INSERT``, or ``DELETE``. If you want your function to be activated on, say, an update or an insertion, write instead
+    ``UPDATE OR INSERT`` in the spot above.
+
+    Declaring the trigger function above is neccessary so that we can then
+    bind the trigger function to the appropiate action on the table. You do not need
+    the body to be blank, but we recommend it to be to avoid code duplication.
+    The blank body will be replaced by the migrator with the completed body in
+    your other file, so do not worry.
+
+    It can be strategic to choose when in your migration you include the code above,
+    as you may want your trigger function binded only after a table is created.
+
+4.  Make sure you include in the down migration section the deletion of the trigger function.
+
+    ```
+        DROP TRIGGER IF EXISTS
+            the_same_name_about_your_trigger_function
+            ON public.the_table_your_function_is_activated_by;
+    ```

navtreedata.js

@@ -199,7 +199,9 @@ var NAVTREE =
             [ "Development Instructions", "/developer/development_instructions/index", [
                 [ "Overview", "/developer/development_instructions/index", null ],
                 [ "Sample Courses Data", "/developer/development_instructions/sample_data", null ],
-                [ "Migrations", "/developer/development_instructions/migrations", null ],
+                [ "Migrations", "/developer/development_instructions/migrations", [
+                    [ "Trigger Functions", "/developer/development_instructions/trigger_functions", null ]
+                ] ],
                 [ "Resolving Merge Conflicts", "/developer/development_instructions/merge_conflicts", null ],
                 [ "Localization / Language Support", "/developer/development_instructions/localization", null ],
                 [ "Continuous Integration Testing", "/developer/testing/index", [