Add handbook docs for extensions, popup, and observable events (#925)

* Add new handbook sections to TOC

Added ObservableEvents, Sextant, MAUI Plugins Popup, and Extensions sections to the handbook table of contents for improved documentation navigation.

* Add handbook docs for extensions, popup, and observable events

Introduced new documentation sections for ReactiveUI Extensions, MAUI Plugins Popup, ObservableEvents, and Sextant. Added index.md and toc.yml files for each topic under the handbook directory to provide comprehensive guides and API references for these libraries.

Author: ChrisPulman

reactiveui/docs/handbook/toc.yml

@@ -48,3 +48,11 @@
   href: when-any.md
 - name: Akavache (v11)
   href: akavache/toc.yml
+- name: ObservableEvents
+  href: observable-events/toc.yml
+- name: Sextant
+  href: sextant/toc.yml
+- name: MAUI Plugins Popup
+  href: maui-plugins-popup/toc.yml
+- name: Extensions
+  href: extensions/toc.yml