Better API and SDKs documentation

[bfirsh]

Background

I have been working on making the Remote API easier to use. This has involved various things:

• Improving the client libraries we provide for Python and Go, and giving them a name to make them sound like a product we provide – the "Docker SDKs". The improvements to the Python API are happening here. Go is a WIP.
• Improving the reference documentation for the Remote API, based on a Swagger definition.
• Providing an overview to the API and SDKs, and some introductory guides to help users get started. (This PR!)

What I've done

I've written some better documentation for the Remote API and SDKs. It includes a better high-level overview, and a new getting started guide.

It also links off to the new Swagger-based API reference. I have moved the links to docs for previous API versions to this page so that users immediately land on something useful.

I've discussed some of the docs with @joaofnfernandes who took a look at a very early version. The version in this PR is not complete yet, but I figure it'd be better to open this sooner rather than later so people can start to take a look and be aware of its existence.

I have left in the old documentation for the time being while we figure out what to do with it.

Todo

• curl examples in the getting started guide.
• A call-to-action at the bottom of the getting started guide ("what's next?")

Dependencies

• The new Python SDK. (Released in 2.0.0.)
• Automatically generated API reference.

Nice-to-haves

• Some improvements to the Go SDK, e.g. a ContainerRun short-hand. (The Go examples in the getting started guide are a bit of a mouthful.)

/cc

• @dnephin for the Go SDK and Swagger stuff.
• @shin- for the Python SDK.

[bfirsh]

I have now based this on top of #606 and updated it to make sense in context of recent changes. It is essentially an extension of that PR, but with:

• an introduction page (engine/api/index.md)
• a getting started guide for various languages and curl (engine/api/getting-started.md)
• a much improved client libraries page (engine/api/sdks.md).

Once #606 is in, I think this is pretty much ready to go. The only hard dependency is Docker SDK for Python 2.0, which will be released before 1.13 anyway. I recall @johndmulhausen saying we might have to review the design of horizontal tabs, because that hasn't been done before. (In fairness, they do look a bit naff, and are hard to use.)

/cc @johndmulhausen @mstanleyjones

[mdlinville]

After #606 is in, you'll need to rebase this again before we can merge it. We're getting there!

[bfirsh]

@mstanleyjones @johndmulhausen Rebased on top of #606 and vnext-engine. Also, the new Docker SDK for Python has been released, so this is all ready to go now. :)

[mdlinville]

I'm going to merge this, but ping @johndmulhausen to do a follow-up PR to toggle all tabs on a page when one is toggled, and set a cookie so the user's preference is remembered on all API pages. I think the function is toggle_all_tabs in alljs.js.

[bfirsh]

Yay thank you! 🎉

Ideally, clicking a tab would toggle all the ones on the page, but I didn't have time to do it in a non-hacky way, so I just left it out.