[Redesign] Support example knobs

[redallen]

Per #1785, some examples would like to be able to have knobs to quickly toggle parts of the example code:

This is problematic with our current examples. It could be supported through a templating system:

```js knobVariant=link
import { Button } from '@patternfly/react-core';

BasicButton = <Button variant={knobVariant} />

and then in Example.js at runtime:
1. Take `knobVariant=link` and look at Button's `variant` type which is `'primary' | 'secondary' | 'tertiary' | 'danger' | 'link' | 'plain' | 'control'` to render a `<select>` dropdown with those options and `link` selected (probably better than radio in the design...)
2. Transform every occurrence of `{knobVariant}` to `"link"`

@mcarrano @rachael-phillips Should the React code change with the variant they select? This seems intuitive to me.

[mcarrano]

Yes. The code should always reflect what's previewed in the example. That's what I've seen in other systems that use knobs.

[redallen]

Once we have listed which of our 100s of examples we want to add knobs to and some methodology of which knobs to display I think this issue can be picked back up.

[mcarrano]

@redallen @evwilkin When we first started down this path. I did spend some time going through the examples and thinking about what props could be turned into knobs in order to reduce the number of unique examples. You can see that in this spreadsheet on the Components definition tab: https://docs.google.com/spreadsheets/d/164a_7itUer1_HOBhI49XUZ-77genlVWBH1Da0F_glxc/edit?usp=sharing

I agree that we need more discussion about this, and would probably want to start with some proof of concept to figure out how we can approach this. @rachael-phillips any further thoughts?

[dlabaj]

No plan to fix. Currently closing issue.