Add regions-zones mapping doc to book

[Prajyot-Parab]

Signed-off-by: Prajyot-Parab [email protected]

What this PR does / why we need it:

• Add regions-zones mapping doc to book

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

Special notes for your reviewer:

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

[netlify]

✅ Deploy Preview for kubernetes-sigs-cluster-api-ibmcloud ready!

🔨 Explore the source changes: f813c6c

🔍 Inspect the deploy log: https://app.netlify.com/sites/kubernetes-sigs-cluster-api-ibmcloud/deploys/6239af2f7eb9080008a6043c

😎 Browse the preview: https://deploy-preview-602--kubernetes-sigs-cluster-api-ibmcloud.netlify.app

[Prajyot-Parab]

/cc @dharaneeshvrd

[k8s-ci-robot]

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[dharaneeshvrd]

lgtm!

[mkumatag]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mkumatag, Prajyot-Parab

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