[docs] Increase clarity around the usage of font icons

[JosephMart]

With this current sample code, I was not able to get SendIcon to work.

I was able to import SendIcon from'@material-ui/core/Send' and use it to display the SendIcon within the button.

• I have followed (at least) the PR section of the contributing guide.

[oliviertassinari]

@JosephMart Thank you for raising the concern, I have added a comment to guide people.

[mbrookes]

@oliviertassinari This is why we eliminated font icon examples from the docs, (other than in the Icon section). They don't work out-of-the box, and cause this kind of confusion.

Any idea why this one is a font icon?

[oliviertassinari]

@mbrookes It demonstrates both solutions can be switched, same output.

[mbrookes]

We already have the Icons section for that, whereas this example has zero discoverability. It seems a bit random to stick a font icon in here.

Your call.

[oliviertassinari]

As long as we have a visual regression test that compares font icons with svg icons, side by side in a button, I'm happy.

[mbrookes]

visual regression test

Aha, right.

If this comes up again, I'll create s dedicated regression test and remove the font icon from the Button examples.