docs: overhaul SOPS documentation for better structure and readability

docs/concepts-and-configurations/sops.md

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+---
+title: SOPS
+---
+
+# SOPS
+
+We're using [SOPS](https://github.com/getsops/sops) for secret management together with the `community.sops.sops` vars plugin for Ansible.
+
+## GPG Keys
+
+In order to edit encrypted files, you need all the GPG public keys imported into your GPG Keychain. You should be able to find the necessary public keys in [infra-secrets](https://git.hamburg.ccc.de/CCCHH/infra-secrets).
+
+## Guides
+
+See the following pages for guidance on how to use SOPS:
+
+- [SOPS: New Host](../guides/sops-new-host.md)
+- [SOPS: Storing Secrets](../guides/sops-storing-secrets.md)