feat: bootstrap Zabbix template repository

Set up a repository for custom Zabbix templates and add the first
template for Dell PowerVault ME storage systems.

- Add conventions, version policy, and release documentation.
- Add validation, index, packaging, export, and import-test scripts.
- Add a disposable Zabbix test stack with version-aware imports.
- Add PowerVault ME SNMP exports for Zabbix 7.0 and 8.0.
- Cover system information, unit health, sensors, ports, and
  firmware revisions.

.github/workflows/validate.yml

@@ -0,0 +1,37 @@
+name: Validate
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+          cache: pip
+          cache-dependency-path: requirements-dev.txt
+      - name: Install dependencies
+        run: pip install -r requirements-dev.txt
+      - name: Lint YAML
+        run: yamllint .
+      - name: Validate manifests
+        run: python3 scripts/validate.py
+      - name: Check README index
+        run: python3 scripts/gen_index.py --check
+
+  import-test:
+    runs-on: ubuntu-latest
+    needs: lint
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v4
+      - name: Import templates into disposable Zabbix
+        run: ./scripts/import_test.sh