Added Katib API reference to the website docs

[janeman98]

What this PR does / why we need it:
Add the Katib API reference to the website docs

Which issue(s) this PR fixes
Fixes #1481

Special notes for your reviewer:
@sarahmaddox
Please review. Please note that I use PyTorchJob as example rather than Metedata reference since the katib reference is the description of each field (like type, description, etc.) which is closer to that in PyTorchJob reference.

Question: there are two TODO in the original document in Github. How should we handle this in the website ? Leave them as TODO or remove them?

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This change is 

[janeman98]

/assign @sarahmaddox

[sarahmaddox]

Preview: https://deploy-preview-1531--competent-brattain-de2d6d.netlify.com/docs/reference/katib/

[sarahmaddox]

Thanks! I've added two suggestions as review comments. One of them is a response to your question about the TODOs in the codel.

[sarahmaddox]

@janeman98 My apologies! I see that now the intermediate _index.md file is gone, there's no way of seeing which version of the API the docs refer to, as the directory no longer appears in the breadcrumbs (see preview).

Please would you put the file back again? content/docs/reference/katib/v1alpha3/_index.md.

(@johnugeorge we should add the version to the API description. I've submitted a PR to suggest this change: kubeflow/katib#1017.)

[janeman98]

@sarahmaddox No problem. I have added the file back : content/docs/reference/katib/v1alpha3/_index.md.

[sarahmaddox]

Thanks @janeman98. Let's wait until my PR kubeflow/katib#1017 is reviewed. If the changes are accepted then we can just manually apply them to this PR (instead of regenerating).

[sarahmaddox]

@janeman98 My PR was merged into the Katib repo. Would you like to apply the same updates manually to your PR? (I can do it if that's easier.) After that, this PR is ready to merge.

[janeman98]

@sarahmaddox Thanks again.
I have applied the same updates manually to this PR. Please review/approve. Feel free to let me know if further modification is needed.

[johnugeorge]

/lgtm

[sarahmaddox]

Thanks @janeman98!
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[janeman98]

Thanks @sarahmaddox !