Add TFJob reference htmls

[richardsliu]

Part of kubeflow/training-operator#731

---

This change is 

[richardsliu]

/assign @sarahmaddox

[sarahmaddox]

Previews:

• https://deploy-preview-516--competent-brattain-de2d6d.netlify.com/docs/references/tfjob/v1beta2/tensorflow/
• https://deploy-preview-516--competent-brattain-de2d6d.netlify.com/docs/references/tfjob/v1beta2/common/

[sarahmaddox]

Thanks! A few comments from me:

• I think you're right about using .md - let's change the file extensions from .html to .md.

• Please change /references/ to /reference/ in the file path, to align with my PR Added the framework for API references. #514

• Please add this markup to the top of each file. I hope this will make Hugo treat the file as a web page that needs the header and navigation chrome around it:

in common.md:

+++
title = "TFJob Common"
description = "Reference documentation for TFJob"
weight = 100
+++

and in tensorflow.md:

+++
title = "TFJob TensorFlow"
description = "Reference documentation for TFJob"
weight = 100
+++

[richardsliu]

@sarahmaddox Thanks! Looking much better now:

https://deploy-preview-516--competent-brattain-de2d6d.netlify.com/docs/reference/tfjob/v1beta2/tensorflow/
https://deploy-preview-516--competent-brattain-de2d6d.netlify.com/docs/reference/tfjob/v1beta2/common/

[sarahmaddox]

Looking much better! Just some broken links - see comments.

[richardsliu]

@sarahmaddox Links should work now.

[sarahmaddox]

Cool, this LGTM. Do we need to hold off until the release date? I won't add an LGTM or approve comment until I hear from you.

[richardsliu]

Couple of options:

1. We can just generate the same documentation for v1beta1 which is already released, or
2. Hold off until 0.5.0 release.

Either way we have to release again when v1beta2 comes along.

I prefer option 1, we are supposed to have API documentation for Beta releases anyway.

[richardsliu]

@sarahmaddox I've pushed the changes to v1beta1. For v1beta2, I've added "in development" to the title since it is not yet released. We can merge this PR if you are ok with it.

[sarahmaddox]

Nice! And actually, the website does reflect master (we've already created the doc archive for 0.4) so it's fine to publish docs related to 0.5 provided we clearly mark them as such.

[sarahmaddox]

/lgtm
/approve

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