Fix broken book links and add section on adding new E2E tests

[salasberryfin]

/kind cleanup

What this PR does / why we need it:

Follow-up to restructuring the book https://github.com/kubernetes-sigs/cluster-api-provider-gcp/pull/1353, as some links were found to be broken.

Additionally, this adds a new Developers section on basic information on how to add a new E2E test case to existing suite.

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

TODOs:

• [x] squashed commits
• [x] includes documentation
• [ ] adds unit tests

Release note:

NONE

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, salasberryfin

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