docs: add comprehensive firewall rules documentation

[barbacbd]

Add detailed documentation for the firewall API feature that allows users to configure GCP VPC firewall rules through the GCPCluster resource.

The documentation covers:

• Default firewall rule management (Managed/Unmanaged policies)
• Custom firewall rule configuration with complete field reference
• Support for ALLOW/DENY rules, Ingress/Egress directions
• Five practical examples including SSH, web traffic, and multi-protocol rules
• Best practices, limitations, and troubleshooting guidance

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Add docs for the Firewall API changes.

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:

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

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Add detailed documentation for the firewall API feature that allows users to configure GCP VPC firewall rules through the GCPCluster resource.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[barbacbd]

/cc @salasberryfin

[barbacbd]

/retest-required

[k8s-ci-robot]

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