[Documentation:Forum] Add markdown equation docs (#704)

There previously were not any docs explaining adding equations to
markdown after the feature was added in
Submitty/Submitty#11931. This PR adds docs
explaining how to embed an equation in any markdownarea on Submitty.

---------

Co-authored-by: Barb Cutler <[email protected]>

Author: lavalleeale

_docs/student/communication/markdown.md

@@ -112,4 +112,12 @@ Then use the standard markdown syntax show below:
 
     ```
 
+* **Equations**  
+    To add an equation using LaTeX, use dollar signs or `\(` and `\)` to enclose the equation. For example:
+
+    ```
+    $E = mc^2$
+    \(E = mc^2\)
+    ```
+
 For more information, see also: [Markdown Guide](https://www.markdownguide.org/)