Update guide to install tools and set up GCP

[jerop]

Update the link to a guide that walks through installing required tools and setting up GCP Project. It previously linked to a docs in KNative repo, which has been deleted. It now links to the relevant guide in KNative official docs.

[tekton-robot]

Hi @jerop. Thanks for your PR.

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[jerop]

/assign @dibyom

[ghost]

/ok-to-test
/approve

[vdemeester]

/lgtm

[tekton-robot]

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