Guide students through posting $ in the forum.

At the moment, text between two dollar signs $ like this $ gets converted to mathjax, which is expected.

However, it makes it difficult to write a sentence like:
Go to a course and add a new post in the forum.
Type: This chair costs $100, but that chair costs $300.
Expected behavior: It should post as two dollar signs with text in between
Actual behavior: There is italics and mathjax formatting between the two dollar signs.

The best workaround I can find is to write:
Type: This chair costs $\$$100, but that chair costs $\$$300.

Acceptance criteria:
There are existing guidelines for inputting special characters in the discussion forums in a wiki in the Demo course. Update that wiki, make sure pointers to it are correct.