docs: add guide on using Cilium to secure CloudNativePG traffic

[zekiyeaydemir]

This contribution adds a new documentation section on securing CloudNativePG-managed PostgreSQL clusters using Cilium, a CNCF Sandbox project that provides eBPF-powered networking and security. It includes an introduction to Cilium, an example CiliumNetworkPolicy manifest to restrict access to PostgreSQL.

Closes #7691

[github-actions[bot]]

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[sxd]

I´m working on adding a couple of stuff here @zekiyeaydemir hopefully will finish tomorrow and I can get a review from you will be awesome! Will let you know!

[mnencia]

/ok-to-merge docs only