Add markdown action

Signed-off-by: Leo Siepel <leosiepel@gmail.com>

Author: lsiepel

.github/markdownlin.yaml

@@ -0,0 +1,37 @@
+# From: https://github.com/openhab/openhab-docs/blob/main/.github/.markdownlint.yaml
+
+default: true
+
+# Expect dash usage for unorderd lists
+MD004:
+  style: dash
+
+# Allow long line lengths
+MD013: false
+
+# Allow duplicate headers for different nesting
+MD024:
+  siblings_only: true
+
+# Allow Multiple top level headers in the same document
+MD025: false
+
+# Allow trailing punctuation in headers
+MD026: false
+MD029:
+  style: one
+
+# Allow inline HTML
+MD033: false
+
+# Code block style
+MD046:
+  style: fenced
+
+# Emphasiszis in underscore
+MD049:
+  style: underscore
+
+# Strong in asterisk
+MD050:
+  style: asterisk

.github/mlc_config.json

@@ -0,0 +1,7 @@
+{
+  "ignorePatterns": [
+    { "pattern": "^https://github.com/openhab/openhab-addons/commit/.*" },
+    { "pattern": "^https://github.com/openhab/openhab-addons/pull/.*" },
+    { "pattern": "^mailto:.*" }
+  ]
+}

.github/workflows/markdownlint.yml

@@ -0,0 +1,39 @@
+name: Check Markdown For Errors
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  markdownlint:
+    name: markdownlint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      # pinning to SHA to mitigate possible supply chain attack
+      - name: Run markdownlint-cli
+        uses: nosborn/github-action-markdown-cli@508d6cefd8f0cc99eab5d2d4685b1d5f470042c1 # v3.5.0
+        with:
+          files: .
+          config_file: ".github/markdownlint.yaml"
+
+  markdown-link-check:
+    name: Broken Links Check
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      # pinning to SHA to mitigate possible supply chain attack
+      - name: Check for broken links
+        uses: tcort/github-action-markdown-link-check@a800ad5f1c35bf61987946fd31c15726a1c9f2ba # v1.1.0
+        with:
+          use-quiet-mode: 'yes' # only show errors in output
+          use-verbose-mode: 'yes' # show detailed HTTP status for checked links
+          config-file: '.github/mlc_config.json'