Add mkdocs.yml JSON schema for markdown extensions (#49)

This allows the Markdown extensions provided by this MkDocs plugin
to be included in the mkdocs.yml JSON schema definition maintained
by the developers of mkdocs-material.

See squidfunk/mkdocs-material#6381 and
squidfunk/mkdocs-material#6378.

Author: AdrianoKF

docs/extensions-schema.json

@@ -0,0 +1,28 @@
+{
+    "$schema": "https://json-schema.org/draft-07/schema",
+    "title": "Neoteroi markdown extensions",
+    "markdownDescription": "https://www.neoteroi.dev/mkdocs-plugins",
+    "oneOf": [
+      {
+        "title": "Cards – Neoteroi Markdown",
+        "markdownDescription": "https://www.neoteroi.dev/mkdocs-plugins/cards/",
+        "const": "neoteroi.cards"
+      },
+      {
+        "title": "Timeline – Neoteroi Markdown",
+        "markdownDescription": "https://www.neoteroi.dev/mkdocs-plugins/timeline/",
+        "const": "neoteroi.timeline"
+      },
+      {
+        "title": "Gantt – Neoteroi Markdown",
+        "markdownDescription": "https://www.neoteroi.dev/mkdocs-plugins/gantt/",
+        "const": "neoteroi.projects"
+      },
+      {
+        "title": "Spantable – Neoteroi Markdown",
+        "markdownDescription": "https://www.neoteroi.dev/mkdocs-plugins/spantable/",
+        "const": "neoteroi.spantable"
+      }
+    ]
+  }
+