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 421994739

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 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