docs: add dataset ORM tutorial page and update navigation with GFM documentation

Add new Dataset ORM tutorial page covering CRUD operations, query operators, schema evolution, and JSON field handling. Update navigation.yaml to include the new tutorial in the tutorials section. Enhance markdown module documentation with GitHub Flavored Markdown support details including tables, task lists, and language identifier attributes for syntax highlighting. Update tutorial index page and pagination links to reflect the new tutorial order. Refactor dataset.wren module source to introduce Sql and QueryBuilder classes with soft-delete support and identifier validation.

test/web/multipart_lf_only.wren

@@ -0,0 +1,15 @@
+// retoor <retoor@molodetz.nl>
+
+import "web" for Request
+
+var body = "------boundary\n" +
+  "Content-Disposition: form-data; name=\"field\"\n" +
+  "\n" +
+  "value\n" +
+  "------boundary--\n"
+
+var headers = {"Content-Type": "multipart/form-data; boundary=----boundary"}
+var request = Request.new_("POST", "/submit", {}, headers, body, {}, null)
+var form = request.form
+
+System.print(form["field"]) // expect: value