Implement Subscription docs

[Foorack]

Current docs should at least cover how to get information about your current subscription. And in the future this has to be expanded when Creator Economy launches. People are very likely to want to script these things.

Available tiers can be fetched with:
GET https://vrchat.com/api/1/subscriptions

And especially interesting, current account subscription can be fetched with:
GET https://vrchat.com/api/1/auth/user/subscription

This allows you to see what kind of subscription you have, when you got it, and when it expires. There is a lot more, but we should start with the basics.

[
	{
		"id":"vrchatplus-yearly",
		"transactionId":"txn_xxxxxxxx-xxxx-4xxx-axxx-xxxxxxxxxxxx",
		"store":"Steam",
		"steamItemId":"5000",
		"amount":9999,
		"description":"VRChat Plus (Yearly)",
		"period":"year",
		"tier":5,
		"active":true,
		"status":"active",
		"expires":"2022-03-04T00:00:00.000Z",
		"created_at":"2020-12-04T13:09:54.344Z",
		"updated_at":"2020-12-10T19:53:45.362Z",
		"licenseGroups":[
			"lgrp_608513da-b213-4e15-80af-bd88c27f0979"
		],
		"isGift":false
	}
]```