Improve documentation for k8s-spark resource template

[theofpa]

What this PR does / why we need it:
Documents the SPARK_K8S_JOB_TEMPLATE_PATH constant with example and reference to the spec.

[feast-ci-bot]

@theofpa: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[feast-ci-bot]

Hi @theofpa. Thanks for your PR.

I'm waiting for a feast-dev member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: theofpa, woop

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[woop]

Thanks @theofpa this is great!

Please remember to sign the commit!