Add docs for creating a simple form

[dmethvin-gov]

I'm not totally comfortable with this because it seems pretty extensive for a "simple" form. In particular:

• If you leave the introduction page out the browser crashes with a "too much recursion" error so it's not really optional. It seems like there's some issue with it redirecting to /introduction which doesn't exist. I expected that if the intro was not specified it would go directly to the first page of the form. I wonder if confirmation is also crash-inducing since we haven't gotten to it in the starter app.

• Even for a simple one-page form you end up having to create wrapper chapters and pages. I'm also not sure if review should be required on a simple one-page form. It's super easy to make mistakes when creating a deeply nested formConfig from scratch and get nonsensical errors because it looks like formConfig isn't checked against a JSON Schema itself. If you miss a level of nesting on something it really confuses the code.

• There is a schema title and a uiSchema ui:title, it isn't clear to me when to use either although I used the uiSchema ones here so I'd have something in that object. However, it seems like using schema title would be a lot less typing and redundancy for this case. There's a similarly confused person who made a ticket in react-jsonschema-form. This particular case makes me think it would be super handy to have the code create a default title from the name of the field, e.g. email becomes Email.

• The page title seems like it could be optional but the React component says that it is not. The React component throws a warning in the console based on propTypes defined.

• The chapter title isn't required (no warnings) but has to be specified in practice. If it's not, the review page will show an empty untitled box in the accordion. Most likely we should require it, unless the review page can also be optional (if so, I don't know how yet).

• When you get to the review page, the review component throws a propTypes warning because the trackingPrefix is not specified, this is related to the undocumented functionality mentioned in Document the ability to track form stats and errors via Google Analytics #134. I don't think we want to make Google Analytics a requirement.

I suspect this will need some changes, either to the text here in the PR, the code, or both. Fire away!

[dmethvin-gov]

Addresses concerns in #79 and #128, maybe others.

[annekainicUSDS]

Generally looks great! One suggestion for some different wording.

All of the issues your mentioned in the PR description are really good for us to document, and I think we should probably create tickets for them to address separately.

[annekainicUSDS]

I might rephrase the opening paragraph here. I feel like the first concepts someone should understand before building a form is that they describe the form in formConfig and they need to render the top-level React component, FormApp, somewhere. I might write:

"In order to build a form, you'll use the configuration file to specify everything your form needs: questions, how questions are grouped together, content on the page, data validation, etc. In the starter app, the file that exports this configuration object is located at /src/js/config/form.js.

Most of the time you'll just be working within the config file to describe the form you need and you won't be writing React components. However, there are 2 cases where you'll need to work with React components directly:

1. Rendering the top-level React component: US Forms System uses a top-level React component, FormApp, to pass the formConfig object, exported from your config file, down to the rest of the components that are rendered. In order for the React component hierarchy to properly render, you'll need to include FormApp in your app's top-level component, and pass it the formConfig object as a prop. See how the starter app does this for an example.
2. Creating custom components: There may be times where you'll want to create your own custom React components to include in the form config. Examples may include creating a custom page you want to render (e.g., an introduction page), creating a component for content (e.g., a warning message) you want to render within a page, or creating a new component to override one of our default ones. In these cases you'll be building React components from scratch."

Something like that?