[MXNET-284] Update release version text to 1.2.0 (#10415)
[MXNET-284] Update release version text to 1.2.0 (#10415)

* Addng a dummy space

* Changing the static references to 1.1.0 to 1.2.0

* Removed the extra space

* In the new page add new two new text sections for for Improved ONNX Support and Introducing Scala

* Updating the text to ensure that Apache goes along with Incubating

* Add rc0 to the release version in the home page

* updated the link to be : https://github.com/apache/incubator-mxnet/releases/tag/1.2.0.rc0.

* Changing the Github 1.2.0.rc0 link in the home page to Confluence

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