feat(docs): add readme for localprocess

[szaher]

What this PR does / why we need it:

add a readme to explain how to use the local-process backend.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[andreyvelich]

I would better move this user instruction to the KF Trainer user guides for now: https://www.kubeflow.org/docs/components/trainer/user-guides/
Similar to KFP, we can have this guide Execute TrainJobs Locally: https://www.kubeflow.org/docs/components/pipelines/user-guides/core-functions/execute-kfp-pipelines-locally/

[andreyvelich]

If you want to add architecture details, we can add Reference section at the bottom of this doc.

[kramaranya]

+1, I think having an example on trainer repo and dedicated page on kubeflow website is sufficient for now
cc @briangallagher