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 <bmcutler@users.noreply.github.com>

### Why is this Change Important & Necessary?

Currently, Submitty's LaTeX rendering only supports in-line math mode
(e.g., `$E=mc^2$`). This makes it difficult to display complex or
multi-line equations in a clear and readable format. This change
introduces support for "display math mode," which renders equations on
their own line and centered, a standard feature in mathematical
typesetting. This enhancement significantly improves the readability of
course materials and student submissions that contain mathematical
formulas.

Closes #11989
Related to #11931

### What is the New Behavior?

**Before:**
Only in-line math using `$...$` or `\(...\)` was rendered. Any
expressions using `$$...$$` or `\[...\]` were displayed as plain text.

**After:**
Users can now use `$$...$$` and `\[...\]` to render mathematical
expressions in display mode. The expressions will appear centered on a
new line. Existing in-line math rendering is unaffected.

**Example:**

**Before:** `$$a^2 + b^2 = c^2$$` would just show as plain text.

**After:** `$$a^2 + b^2 = c^2$$` will render as:

$$
a^2 + b^2 = c^2
$$

### What steps should a reviewer take to reproduce or test the bug or
new feature?

1. Navigate to an area in Submitty that renders Markdown, such as the
instructions for a gradeable or a post in the discussion forum.
2. Create or edit a post and enter a mathematical expression using
display mode delimiters. For example: `$$f(x) = \sum_{n=0}^{\infty}
\frac{f^{(n)}(a)}{n!} (x-a)^n$$`
3. Also test the other display mode delimiter: `\[ \int_a^b f(x) \, dx =
F(b) - F(a) \]`
4. Add an in-line expression to ensure existing functionality is not
broken: `This is an in-line equation: $E=mc^2$.`
5.  Save and view the rendered output.
6. **Expected Result:** The equations wrapped in `$$` and `\[` should be
rendered on their own lines and centered. The equation wrapped in `$`
should appear in-line with the text.

### Automated Testing & Documentation

* **Automated Testing:** The existing front-end tests that check for
KaTeX rendering should be updated to include test cases for display math
mode. A new GitHub issue should be created if these tests are not
included in this PR.
* **Documentation:** The documentation on
[submitty.org](http://submitty.org) regarding Markdown usage and
supported LaTeX syntax should be updated to include information about
the newly supported display math delimiters (`$$...$$` and `\[...\]`). A
new GitHub issue should be created to track this documentation update.

### Other information

* **Breaking Change:** No, this is not a breaking change. It is additive
and preserves all existing functionality for in-line math rendering.
* **Migrations:** This PR does not include database migrations as it is
a front-end configuration change.
* **Security Concerns:** There are no known security concerns with this
change. It involves modifying the configuration of the KaTeX library,
which is designed to safely render mathematical expressions without
executing arbitrary scripts. The change does not introduce new libraries
or dependencies. render mathematical expressions without executing
arbitrary scripts. The change does not introduce new libraries or
dependencies.

---------

Co-authored-by: Samarth Naik <samarthnaikk@gmail.com>
Co-authored-by: Sátvik Karanam <89281036+skara9@users.noreply.github.com>