Initial website documentation for Java API

[andrewfayres]

Description

This is the first pass at creating the website documentation needed for the new Java API release.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-13289/10/index.html
  http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-13289/10/install/java_setup.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[kalyc]

@mxnet-label-bot add [pr-awaiting-review]

[kalyc]

@andrewfayres thanks for your contribution! Have these instructions been tested locally?

[aaronmarkham]

Please add a preview link to this in your description:
http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-13289/3/install/java_setup.html

[andrewfayres]

Will add new link once build finishes

[aaronmarkham]

In addition to my previous comments, let's handle the tutorials main index page and the API navigation in one step.

• Please add a Java section to the /docs/tutorials/index.md and link to the tutorials
• Please add Java to the navigation under API. I talk about the steps to do this here: https://cwiki.apache.org/confluence/display/MXNET/Website+Updating+Guide
• Please add /docs/tutorials/java/index page (using glob approach I mentioned in my review)

Let me know if I can assist.

[andrewfayres]

I think I've addressed all feedback. As soon as the build finishes and we get the preview version I'll update the links and double check everything.

[aaronmarkham]

LGTM... please review the preview when it finishes though.

[lanking520]

@andrewfayres Please confirm this before we merge it.

[lanking520]

LGTM