Full screen snapshot images have not alt text or accessible labels

[nicolethoen]

When generating screenshots for full screen demos or examples, we need to also generate an accessible label as alt text - or point to the example/demo heading as it's label.

This will fix MANY of the website a11y issues.

[evwilkin]

Full screen <img> is generated in example.js and should be able to reference the title prop used in the anchor tag wrapping the image.