Quantum Engine documentation

[dstrain115]

• Documentation for engine:
  -- New Concepts page
  -- Getting started tutorial
  -- Reservation colab
  -- Colab template including ipython download instructions.

[balopat]

There are a bunch of readthedocs.io references here that will be broken on devsite.
I also found some nits but I think we can do the thorough review after these are in.

git grep readthedocs.io actually shows a bunch (even more occurrences than this PR, I'll have to fix those up separately):

docs/google/concepts.ipynb:    "Calibration metrics and can be retrieved via the console or via cirq calls.  See more details on the [Calibration Metrics](https://cirq.readthedocs.io/en/latest/docs/google/calibration.html) page.\n",
docs/google/concepts.ipynb:    "You can use this [reservation colab utility](https://cirq.readthedocs.io/en/latest/docs/tutorial/google/reservations.ipynb)  to view the processor schedule and make reservations."
docs/google/concepts.ipynb:    "[Getting started colab](https://cirq.readthedocs.io/en/latest/docs/tutorials/google/start.html)\n",
docs/google/concepts.ipynb:    "Learn how to use the [Engine class](https://cirq.readthedocs.io/en/latest/docs/google/engine.html) in cirq.\n",
docs/google/concepts.ipynb:    "Learn more about the [devices and gate sets](https://cirq.readthedocs.io/en/latest/docs/google/specification.html) available on the devices.\n",
docs/google/concepts.ipynb:    "[Download a ipython template](https://cirq.readthedocs.io/en/latest/docs/google/colab.html) to use in colaboratory\n",
docs/tutorials/google/colab.ipynb:        "* [Quantum Engine concepts](https://cirq.readthedocs.io/en/latest/docs/google/concepts.html)\n",
docs/tutorials/google/colab.ipynb:        "* [Quantum Engine documentation](https://cirq.readthedocs.io/en/latest/docs/google/engine.html)\n",
docs/tutorials/google/colab.ipynb:        "* [Cirq documentation](https://cirq.readthedocs.io)\n",
docs/tutorials/google/reservations.ipynb:        "For information on how to download a colab from github, see [these instructions](https://cirq.readthedocs.io/en/latest/docs/google/colab.html)"
docs/tutorials/google/start.ipynb:        "You can find more about running this in colaboratory in the [Colab documentation](https://colab.sandbox.google.com/notebooks/welcome.ipynb) or in our cirq-specific guide to [running in colab](https://cirq.readthedocs.io/en/latest/docs/google/colab.html)."
docs/tutorials/google/start.ipynb:        "Now that you've enabled Quantum Engine and configured the notebook, let's create a basic program with Cirq. After reviewing the code, **run this block** to run a circuit, and print a circuit diagram and results. To learn more, refer to the [Intro to Cirq](../basics.ipynb) and [Cirq documentation](https://cirq.readthedocs.io)."
docs/tutorials/google/start.ipynb:        "Approved users can access quantum hardware in two modes. First, all approved users have access to a processo in \"open-swim\" which is a first-in-first-out queue with fairness algorithm that balances jobs across users in the queue. Secondly, processors can be reserved in hourly blocks if the user is approved.  You can learn more about the reservation system on the [concepts page](https://cirq.readthedocs.io/en/latest/docs/google/engine.html). We'll use the processor `mcgee` in this demo."
docs/tutorials/google/start.ipynb:        "* Use [this template colab](https://cirq.readthedocs.io/en/latest/docs/google/colab.html) as a base for your own explorations.\n",
docs/tutorials/google/start.ipynb:        "* Explore [best practices](https://cirq.readthedocs.io/en/latest/docs/google/best_practices.html) for getting circuits to run on hardware.

[balopat]

See #3294 for fixing the readthedocs.io URLs as an example

[balopat]

LGTM - let's get it in. I'll do a more thorough read later, we can iterate on nit picking.

[dstrain115]

LGTM - let's get it in. I'll do a more thorough read later, we can iterate on nit picking.

Sure, feel free to file bugs or ping me if you notice issues in documentation and I can fix them later.