Provide docs for 3.7

[Orta Therox (orta)]

This PR effectively wraps orta/typescript-play#21 with the website infra to make it work.

It adds a series of smaller docs on different features in TypeScript built to complement the release notes in the playground environment. We can link to these in the main release notes (and/or edit the beta post)

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Typos for README.md

Line	Typo
11	monaco/IDE-like environment which ha
11	-like environment which has a TSServer running for

Typos for Mixins.ts

Line	Typo
3	// Mixins are a faux-multiple inheritan
76	// functions due to their mixins:

Typos for generateTOC.js

Line	Typo
10	s from a given folder and its subfolders. */
112	whatisnew: true
131	"Fixits"
149	// Ensure all subfolders are in the sorted section

Typos for Big number literals.ts

Line	Typo
16	o convert these numbers to BigInts instead
21	/ TypeScript will now offer a fixit for number

Typos for Flattened Error Reporting.ts

Line	Typo
19	// Now they're inline. :tada:

Typos for Nullish Coalescing.ts

Line	Typo
3	// The nullish coalescing operator is an alt
41	// You can read more about nullish coalescing in the 3.7 blog po

Typos for Assertion Functions.ts

Line	Typo
60	// This will then cut the enum's possible states to: 1

Typos for Recursive Type References.ts

Line	Typo
31	name: "Highcrest",

Got false positives?

Make changes to the global settings spellcheck.json in /artsy/peril-settings.

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