The text in `cue help` started by explaining custom CUE commands
as supported by `cue cmd`, which is a relatively advanced feature
which confuses nearly every new user to CUE.

We have a much nicer and more up to date introduction to CUE
on the landing page of cuelang.org, so borrow some text from there,
and link to the website for more info and docs.

The README has a similar story in that it does contain relevant info,
but it is pretty out of date at this point, as well as verbose.
For example, it spends two full sections on scripting and tooling,
which are only one of the many aspects of CUE.
And, as before, it doesn't introduce CUE as nicely as cuelang.org does.

Use exactly the same text that we've now added to the top of `cue help`,
but using markdown formatting for the link to cuelang.org instead.

Also add a badge next to the Go docs one to surface the presence
of our website documentation section at the top of the README,
adapted from Jonathan Matthews' CL 1202912.

We also align the top heading in the README to match the title
and heading seen when one visits cuelang.org.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I0f6cdbf6b181acfddbdcc0ea3a8ffc8e102d8101
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1203113


TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Jonathan Matthews <github@hello.jonathanmatthews.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.io>