docs(xychart): add README, add docs/xychart page, add XYChart gallery tile

[williaster]

📝 Documentation

This PR

• fleshes out the README for @visx/xychart to cover most aspects of the API, including some example code snippets
  • note I've added <details /> sections so that the rest of the /docs/xychart page with example tile(s) + comprehensive API will be visible without needed to scroll a lot
• adds a /docs/xychart page to the demo site, and the XYChartTile to the gallery 🎉

I plan to add a couple more examples but non-gallery codesandboxes will be easier to create when the package is published

@kristw @hshoff

[coveralls]

Pull Request Test Coverage Report for Build 422008542

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.258%

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Totals
Change from base Build 413784565:	0.0%
Covered Lines:	1732
Relevant Lines:	2694

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💛 - Coveralls

[williaster]

hmm, may not be able to cleanly create the @visx/xychart/animated index, right now I think because we have main: lib/index.js for the package, and that index doesn't reference lib/animated/index.js we don't get the correct types, etc. (which is failing the build)

[williaster]

Going to merge this since it's been open for a while and is just docs w(hich are blocking 1.3.0 as it stands), happy to address any feedback in a following PR