docs(renovate): add custom managers documentation

.renovate/CUSTOM_MANAGERS.md

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+# Renovate Custom Managers
+
+This document describes the custom regex managers configured for the boilerplates repository.
+
+## 1. Docker Compose Templates
+
+**File Pattern:** `library/compose/**/*.j2`
+
+**Detects:** Docker images in compose files
+
+**Example:**
+```yaml
+services:
+  app:
+    image: ghcr.io/goauthentik/server:2025.6.3
+    # Renovate will detect: depName=ghcr.io/goauthentik/server, currentValue=2025.6.3
+```
+
+## 2. Kubernetes Helm Values
+
+**File Patterns:** 
+- `library/kubernetes/**/helm/values.yaml`
+- `library/kubernetes/**/*.j2`
+
+**Detects:** Docker images using repository + tag pattern (common in Helm charts)
+
+**Example:**
+```yaml
+image:
+  repository: "longhornio/longhorn-engine"
+  tag: "v1.9.1"
+# Renovate will detect: depName=longhornio/longhorn-engine, currentValue=v1.9.1
+```
+
+## 3. Terraform Providers
+
+**File Patterns:**
+- `library/terraform/**/*.tf`
+- `library/terraform/**/*.j2`
+
+**Detects:** Terraform provider versions
+
+**Example:**
+```hcl
+terraform {
+  required_providers {
+    proxmox = {
+      source  = "telmate/proxmox"
+      version = "3.0.1-rc9"
+    }
+  }
+}
+# Renovate will detect: depName=telmate/proxmox, currentValue=3.0.1-rc9
+```
+
+## 4. Terraform Modules
+
+**File Patterns:**
+- `library/terraform/**/*.tf`
+- `library/terraform/**/*.j2`
+
+**Detects:** Terraform module versions from Git sources with `?ref=` parameter
+
+**Example:**
+```hcl
+module "vpc" {
+  source = "git::https://github.com/terraform-aws-modules/terraform-aws-vpc.git?ref=v5.1.2"
+}
+# Renovate will detect: depName=github.com/terraform-aws-modules/terraform-aws-vpc, currentValue=v5.1.2
+```
+
+## Post-Upgrade Tasks
+
+After any dependency update, Renovate runs `.renovate/sync-template-version.sh` which:
+1. Detects which `template.yaml` files were affected by the update
+2. Automatically bumps their patch version
+3. Includes the updated `template.yaml` files in the Renovate PR
+
+This ensures template metadata stays in sync with dependency updates across all modules (compose, kubernetes, terraform).