📖 docs: add links to capg book

[salasberryfin]

What this PR does / why we need it:

The CAPG docs are now available as a book at https://cluster-api-gcp.sigs.k8s.io/. This PR updates links in the CAPI book to point to the new CAPG book.

Which issue(s) this PR fixes:
Fixes #

/area documentation

[k8s-ci-robot]

Hi @salasberryfin. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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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[killianmuldoon]

/ok-to-test
/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: d18e255901c1846418edf611dad555a2f2249820

[chrischdi]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrischdi

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