update contributing guide for trainer v2

[burhanuddin6]

What this PR does / why we need it: Updates the Contributing Setup and Testing Guide from V1 to Kubeflow Trainer V2. Replaces older instructions by correct instructions for each section in the original Contributing.md file Checklist:

• [ ] Docs included if any changes are user facing

[burhanuddin6]

Update the `newTag` key in `./manifests/overlayes/standalone/kustimization.yaml` with the new image.

I think this is unnecessary and should be removed

[burhanuddin6]

@andreyvelich @franciscojavierarceo @Electronic-Waste review requested. I see the other PRs have some parts missing which I have done

[coveralls]

Pull Request Test Coverage Report for Build 14018198395

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 64.403%

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Totals
Change from base Build 13991199064:	0.0%
Covered Lines:	1688
Relevant Lines:	2621

---

💛 - Coveralls

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[burhanuddin6]

closing this PR since a new PR has been made here which rebases to the current updated CONTRIBUTING.md.