docs: improve custom certificate installation guide

[dlanov]

Summary

Improve the custom control-plane certificate installation guide by making it a complete, directly executable workflow.

This update:

• Links to the cert-manager kubectl installation instructions.
• Creates the envoy-gateway-system namespace before applying namespaced resources.
• Corrects the envoy-gatewy Secret typo.
• Adds readiness checks for the CA issuer and certificates.
• Verifies the generated TLS Secrets.
• Adds a Helm values example and installation command.
• Explains that the certgen job should remain enabled because it preserves existing custom certificate Secrets.
• Adds deployment, pod, and Helm release verification commands.
• Documents how to keep the Kubernetes cluster domain aligned with the controller certificate DNS names.

Validation

• tools/node_modules/.bin/markdownlint -c .github/markdown_lint_config.json site/content/en/latest/install/custom-cert.md
• cd site && npm run build:production
• make docs-check
• git diff --check

Fixes #5757.

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