api/disk-templates: adopt directory structure from clan modules

templates/disk/single-disk/README.md

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+---
+description = "Simple single disk schema"
+---
+# Description
+
+This schema defines a GPT-based disk layout.
+
+### **Disk Overview**
+
+- **Name**: `main-{{uuid}}`
+- **Device**: `{{mainDisk}}`
+
+### **Partitions**
+
+1. **EFI System Partition (ESP)**
+   - Size: `500M`.
+   - Filesystem: `vfat`.
+   - Mount Point: `/boot` (secure `umask=0077`).
+
+2. **Root Partition**
+   - Size: Remaining disk space (`100%`).
+   - Filesystem: `ext4`.
+   - Mount Point: `/`.