Chore/remove redundant demos

[ryami333]

A big re-jig of the demo page and a couple of little updates to the getting-started example boilerplate.

Notes:

• Fancy styles have been updated to put the arrow on the left rather than the right. They were dependent on a block-level descendant of AccordionItemHeading, which as we now know, is not permitted (only "phrasing content" is allowed inside a button).
• Similarly, all heading tags nested inside AccordionItemHeading have been removed in both the demo and the README.
• Removed the react react-dom from the install line in the README. It's a bit redundant - if people don't already have React installed then they're in the wrong place.
• Used 'placeholder' content for accordion items on the demo page. It was a bit of a confusing mess to have content copied from the README in there, and a maintenance risk too.
• Remove examples from the demo page which weren't strictly supported and reflected in the README. Moving away from supporting users' custom/bespoke solutions.
• Made the demo app itself much more DRY. Abstracted away re-used parts (like the AccordionItems and the Arrow).

[ryami333]

There's still a TODO on here, and the render-prop one will need updating too. But no dramas - only merging to a WIP branch.

[samanthaksanders]

I'm not sure, was it valuable to show that you can put accordions inside accordions?

I'd also make sure that we still have some demo content that has a link or two inside the panel so we can demonstrate that tabbing to those will still work.

[ryami333]

Yeah I would consider re-instating the nested one. My thinking was that this demo could afford to be a lot more minimal, and that by showing too many different patterns it was becoming overwhelming.

[samanthaksanders]

Yes, good idea re simplifying.

[ryami333]

Links are a good idea... I'll whack some of those in there.

[ryami333]

Links there now @samanthaksanders. Going to merge now, and will revisit the nested accordion when I come back to do the 'TODOs'

[coveralls]

Coverage remained the same at 98.99% when pulling 6811452 on chore/remove-redundant-demos into 7a05caf on next.