fix: add missing temporal-cloud-encryption doc

[mervivian]

Summary

Five places across four docs (index.md, deploy-operator.md, deploy-helm.md, temporal-hosting.md) link to temporal-cloud-encryption.md, but the file doesn't exist. This leaves any user trying to set up a custom Temporal payload codec (e.g. AWS KMS) with no guidance. The new file documents both the built-in AES-256-GCM approach and how to write, package, and configure a custom Python codec class.

How to reproduce

Click any link to temporal-cloud-encryption.md in the docs — index.md line 57, deploy-operator.md line 121, deploy-helm.md line 231, or temporal-hosting.md lines 36 and 109 — and get a 404.

Test plan

Verify that all five cross-references now resolve to the new file, and that the kubectl create configmap command and the CR/values snippets match the patterns already shown in acme-temporal-cloud-custom-codec.yaml.