Migrate to using docusaurus, rather than docsify for documentation (#603)

* Add docusaurus documentation (to replace the docsify setup
* Remove older docs
* Specify documentation as the gh pages build action working directory

documentation/docs/features/_category_.json

@@ -0,0 +1,8 @@
+{
+  "label": "Features",
+  "position": 3,
+  "link": {
+    "type": "generated-index",
+    "description": "Features supported by Khoj"
+  }
+}