Set up validation github workflow

.github/dependabot.yml

@@ -0,0 +1,14 @@
+# dependabot
+# Ref: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+# ------------------------------------------------------------------------------
+version: 2
+updates:
+- package-ecosystem: github-actions
+  directory: /
+  schedule:
+    interval: monthly
+  groups:
+    # open a single pull-request for all GitHub actions updates
+    github-actions:
+      patterns:
+      - '*'

.github/workflows/validation.yml

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+name: Validate TEI documents
+
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    paths:
+      - "tei/*.xml"
+
+jobs:
+  validate_tei:
+    runs-on: ubuntu-latest
+    name: Validate against TEI_all
+    steps:
+      - name: Checkout
+        uses: actions/checkout@9bb56186c3b09b4f86b1c65136769dd318469633 # v4.1.2
+      - name: Validate
+        uses: dracor-org/tei-validate-action@94b08b8bf0dd1810fe7962aa59435b46c769bbcb # v1.2.0