[vcpkg manifest] Add documentation!

[strega-nil]

This adds documentation for manifest files and for manifest mode. It also converts the existing specification of the supports field to an EBNF, since I think that's easier to read and more exact.

There are two changes made to vcpkg manifests themselves as part of this documentation update:

• $.features is now additionally allowed to be an object, and x-format-manifest will rewrite to this new object syntax.
• x-format-manifest is now a non-experimental command

Example:

{
  "features": [
    {
      "name": "foo",
      "description": "bar"
    }
  ]
}

gets rewritten to

{
  "features": {
    "foo": {
      "description": "bar"
    }
  }
}

[BillyONeal]

I'm not done reviewing yet; the thing I'm "request changes" over is the documentation of x- commands. In the case of x-set-installed IMO the previous explicit install/remove instructions were better for demonstrating how the tool works. In the case of x-format-manifest or similar, if we want users to use them, we should remove the x-.

[BillyONeal]

Thanks!