Explore possible ways for a "Help" button or "Intro" to FSH Online to work

Transferred from internal JIRA issue originally created 2021-05-10.

Possible ideas that have been discussed:

* Static overlay of tooltip/blurbs that point out different areas of the app and what each section/button does
* An interactive introduction to the tool that walks through step by step the basic workflow of the tool. This could include steps that say "Enter the following example FSH: ..." and could point to "Click the 'Convert to JSON' button".
* A simple modal dialog with instructions for how to use the tool
* A page in FSH School that documents FSH Online

Start by reviewing the four proposed approaches in the issue and inspecting the current FSH Online interface to understand where help could fit. Compare the overlay, interactive introduction, modal, and FSH School documentation options, then define which approach is selected and what user guidance it must provide before implementation is considered done.

Written by the indexing model from the issue text.