Add jest tests and CI workflow

__tests__/buildNav.test.js

@@ -0,0 +1,15 @@
+const { buildNav } = require('../src/generator');
+
+test('generates navigation tree', () => {
+  const pages = [
+    { file: 'guide/install.md', data: { title: 'Install', order: 1 } },
+    { file: 'guide/usage.md', data: { title: 'Usage', order: 2 } },
+    { file: 'guide/nested/info.md', data: { title: 'Info', order: 1 } }
+  ];
+  const tree = buildNav(pages);
+  const guide = tree.find(n => n.name === 'guide');
+  expect(guide).toBeDefined();
+  expect(guide.children.length).toBe(3);
+  const install = guide.children.find(c => c.name === 'install.md');
+  expect(install.path).toBe('/guide/install.html');
+});