Switch to a YAML markdownlint configuration file

This converts the existing `.mdl_config.json` file to an equivalent
`.mdl_config.yaml` file. The reference in the markdownlint pre-commit hook
configuration is updated to match.

Co-authored-by: Shane Frasier <jeremy.frasier@trio.dhs.gov>

.mdl_config.json

@@ -1,25 +0,0 @@
-{
-  "MD003": {
-    "style": "atx_closed"
-  },
-  "MD004": {
-    "style": "dash"
-  },
-  "MD013": {
-    "code_blocks": false,
-    "tables": false
-  },
-  "MD024": {
-    "allow_different_nesting": true
-  },
-  "MD029": {
-    "style": "one"
-  },
-  "MD033": {
-    "allowed_elements": [
-      "h1",
-      "img"
-    ]
-  },
-  "default": true
-}

.mdl_config.yaml

@@ -0,0 +1,24 @@
+---
+
+default: true
+
+MD003:
+  style: "atx_closed"
+
+MD004:
+  style: "dash"
+
+MD013:
+  code_blocks: false
+  tables: false
+
+MD024:
+  allow_different_nesting: true
+
+MD029:
+  style: "one"
+
+MD033:
+  allowed_elements:
+    - h1
+    - img

.pre-commit-config.yaml

@@ -35,7 +35,7 @@ repos:
     hooks:
       - id: markdownlint
         args:
-          - --config=.mdl_config.json
+          - --config=.mdl_config.yaml
   - repo: https://github.com/pre-commit/mirrors-prettier
     rev: v2.3.0
     hooks: