Add 1.5 to version dropdown.

[shannonbradshaw]

Fixes #3133

[shannonbradshaw]

/hold

Need to cut the 1.5 website branch first.

[shannonbradshaw]

/hold cancel

[shannonbradshaw]

@annajung @kimwnasptd @jbottum

[annajung]

/lgtm

As a side note, I think it would be helpful to think about removing past versions every time there is a new version added. Maybe keep the previous 5 or 6 versions (from v1.0 - v1.5) and remove the rest. Are there users who actually access v0.2 docs? We can discuss this in the retro as well.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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[jbottum]

@shannonbradshaw FYI, I just tried this and got this error,

This site can’t be reachedCheck if there is a typo in v1-5-branch.kubeflow.org.
DNS_PROBE_FINISHED_NXDOMAIN

[shannonbradshaw]

Still need to cut the branch. Waiting on one more PR.