Add external links that make some content easier to discover

[Bobgy]

/assign @rmgogogo
/assign @IronPan

Idea was proposed in #1253
UPDATE, latest screenshots:

The three links are:

• github repo: https://github.com/kubeflow/pipelines
• aihub samples: https://aihub.cloud.google.com/u/0/s?category=pipeline
• documentation: https://www.kubeflow.org/docs/pipelines/

Also added compile pipeline documentation link in pipeline upload dialog:

The link is: https://www.kubeflow.org/docs/pipelines/sdk/build-component/#compile-the-pipeline

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This change is 

[Bobgy]

Fixes #1253

[Bobgy]

video: https://drive.google.com/file/d/1HU76mAjDFCCRCz_QLiCaWs0_f0YF8UM9/view
This is my initial attempt, I didn't go for the design that collapse all three external links to a single "..." item because it needs some complex animation set up to make that fluent.

Any feedback?

[rmgogogo]

/lgtm

[rmgogogo]

hi Yuan, I just notice this PR.
Is it still on radar to be continued? Seems put there for days.

I'm generally OK. We have another working item which link the KFP with explicit cluster in GCP. It may require more changes and we can handle it in another PR.

/lgtm

[Bobgy]

Yes, I plan to finish this. After other things I've put a higher priority.

[Bobgy]

FYI, I got the github icon from https://github.com/logos
It's recommended to be used when placed next to a link to github.

[Bobgy]

/retest

[Bobgy]

/retest

[rmgogogo]

/lgtm

Thanks.

[Bobgy]

/approve

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