Docs: Adds new button examples

[FrederikSchlemmer]

Examples for the new buttons (stroked & flat) where missing in the documentation.

Fixes #11974

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[FrederikSchlemmer]

I signed it!

[googlebot]

CLAs look good, thanks!

[josephperrott]

Can you move the stroked and flat buttons should be above the icon and fab buttons.

[FrederikSchlemmer]

@josephperrott Thanks for the review! Finished your requested changes.

[jelbourn]

This copyright doesn't have to go here; it's added automatically from the docs site

[FrederikSchlemmer]

Okay thanks for the review! Copyright is removed!

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