Support one level of dropdown menus in the navbar

A `main` menu entry with child entries (others with `parent:` set) is
now rendered as a Bootstrap dropdown; flat entries are unchanged.
Active-state highlighting is trailing-slash tolerant and marks the
dropdown parent active when one of its children is the current page.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -34,6 +34,37 @@ Optional params: `title`, `height` (a CSS length such as `60vh`, which
 crops the image with `object-fit: cover`) and `class`.
 
 
+## Navigation
+
+The navbar renders the `main` menu. A top-level entry that has child
+entries (other entries pointing at it with `parent:`) is rendered as a
+Bootstrap dropdown; entries without children stay as plain links. Only
+one level of nesting is supported (Bootstrap 5 has no nested submenus).
+
+```yaml
+# hugo.yaml / menus.yaml
+menu:
+  main:
+    - name: Home
+      pageRef: /
+      weight: 1
+    - name: Research
+      weight: 2
+    - name: Proposals
+      parent: Research
+      pageRef: /proposals
+      weight: 1
+    - name: History
+      parent: Research
+      pageRef: /history
+      weight: 2
+```
+
+The entry matching the current page (and its parent, for a dropdown
+child) gets Bootstrap's `active` class. A parent entry acts only as the
+dropdown toggle, so it does not need its own `url`/`pageRef`.
+
+
 ## Custom error page
 
 The theme has a custom 404 error page.