ND4J: Add instructions for AVX artifacts

[crockpotveggies]

This issue has been migrated from deeplearning4j/deeplearning4j#6574

Original author @saudet

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The instructions to use AVX binaries on the following page isn't available in the new docs:
https://github.com/deeplearning4j/nd4j/blob/gh-pages/getstarted.md

[crockpotveggies]

@saudet any idea where we're at with this?

[saudet]

What do you want me to do? Feel free to move the content to wherever it should be. Why didn't that page get moved in the first place?

[saudet]

The content of this page also seems to be missing from the new site:
https://github.com/deeplearning4j/nd4j/blob/gh-pages/dependencies.md

[treo]

@crockpotveggies can we get this sorted before beta4 release?