docs/reference: add first cut of spec

Ultimately requires script to keep in sync with go.mod version of
cuelang.org/go.

For now we vendor in a copy by hand from v0.6.0.

The diff compared to that commit is:

    ----
    -title: "The CUE Language Specification"
    -authors:
    -- mpvl
    ----
    +<!--
    + Copyright 2018 The CUE Authors
    +
    + Licensed under the Apache License, Version 2.0 (the "License");
    + you may not use this file except in compliance with the License.
    + You may obtain a copy of the License at
    +
    +     http://www.apache.org/licenses/LICENSE-2.0
    +
    + Unless required by applicable law or agreed to in writing, software
    + distributed under the License is distributed on an "AS IS" BASIS,
    + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + See the License for the specific language governing permissions and
    + limitations under the License.
    +-->
    +
    +# The CUE Language Specification

We drop the h1 title because that is covered in the front matter, and
the copyright is covered by the top-level COPYRIGHT file in the
cuelang.org repo.

Signed-off-by: Paul Jolly <[email protected]>
Change-Id: I441a5de1fa9b3df365c5c59abd71d20cee494ef5
Preview-Path: /docs/reference/spec/
Dispatch-Trailer: {"type":"trybot","CL":1167703,"patchset":9,"ref":"refs/changes/03/1167703/9","targetBranch":"alpha"}