Added the framework for API references.

[sarahmaddox]

I've added an index page for API references, in preparation for the TFJob reference described in issue kubeflow/training-operator#731

I've also added a link from the TFJob user guide to the (upcoming) reference, and configured a compact left-hand nav to make it easier to see all the doc sections.

Note that this PR isn't ready to submit, as it assumes the existence of the v1beta2 API reference for TFJob.

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This change is 

[sarahmaddox]

Preview:

• https://deploy-preview-514--competent-brattain-de2d6d.netlify.com/docs/reference/
• https://deploy-preview-514--competent-brattain-de2d6d.netlify.com/docs/components/tftraining/#more-information

[sarahmaddox]

/assign @richardsliu
/uncc @gaoning777
/uncc @lluunn

[sarahmaddox]

/hold

Holding this one until I can merge it with the references in #516

[sarahmaddox]

Fixes #519

[richardsliu]

/lgtm

[sarahmaddox]

Preview: https://deploy-preview-514--competent-brattain-de2d6d.netlify.com/docs/reference/overview/

The ordering of the pages is still a little weird, but I think it's good enough to commit. I'll continue working on the ordering when we add additional reference docs.

[sarahmaddox]

/hold cancel

[sarahmaddox]

@richardsliu Please would you review and approve if you're happy with the changes.

[richardsliu]

Looks great.
/lgtm
/approve

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