Docs: listed kubectl as a pre-requisite

[vjayaramrh]

This PR updates the document to list kubectl as a pre-requisite that needs to be installed in addition to the other pre-requisites listed.

Fixes #3987

[google-cla]

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[natasha41575]

Thanks for the PR! You will need to sign the Google CLA for us to be able to merge this PR. You can see details here: https://github.com/GoogleContainerTools/kpt/pull/3988/checks?check_run_id=14119644699

[vjayaramrh]

@natasha41575 FYI I signed the CLA a month back

[natasha41575]

The CLA check is still failing. You can find details here: https://github.com/GoogleContainerTools/kpt/pull/3988/checks?check_run_id=14119644699, and there are some tips on troubleshooting if you've already signed the CLA.

[vjayaramrh]

The CLA check is still failing. You can find details here: https://github.com/GoogleContainerTools/kpt/pull/3988/checks?check_run_id=14119644699, and there are some tips on troubleshooting if you've already signed the CLA.

@natasha41575 Thanks