Reorganizing high level docs structure and adding fork me on github ribbon

[sandeep-krishnamurthy]

• Re-organized high level docs structure.
• Added new headers - Community, Deep Learning Concepts
• Renamed back "Supported Languages" to "API" because all tutorials from
  this section will move out to tutorials section.

[pluskid]

I feel "deep learning concepts" are more about dl system / implementation concepts. I think it should be "DL System Design" or just "MXNet Internals" @tqchen @mli ?

[sandeep-krishnamurthy]

@pluskid @piiswrong @mli
Initial idea was to move away general deep learning system design concepts from Architecture and have a separate page for it.
However, I think your suggestions are valid to keep them together. So users first read basic DL design concepts then read MXNet architecture followed by references to MXNet modules.

I am taking up re-organizing Architecture page next, I will remove "Deep Learning Concepts" header in that change. Can we merge this PR as it contains other important changes like fork me ribbon, "community" new heading and renaming supported languages to "API"?

[piiswrong]

http://mxnet.dmlc.ml/
ribbon overlapped with language selection.

[sandeep-krishnamurthy]

I observe that we need to fix CSS issue in header. Created the issue #3531 with details on other issues in header and possible fixes.