Add docs for observables and operators in ReactiveUI (#926)

* Add Observables and Operators to TOC

Added 'Observables Introduction' and 'Operators' sections to the reactive programming table of contents for improved navigation.

* Add docs for observables and operators in ReactiveUI

Introduced two new documentation files: one providing an introduction to observables, their lifecycle, creation, and best practices; and another detailing common reactive operators, their usage, and best practices in ReactiveUI.

Author: ChrisPulman

reactiveui/docs/reactive-programming/toc.yml

@@ -1,4 +1,8 @@
 - name: Reactive Programming
   href: index.md
+- name: Observables Introduction
+  href: observables.md
+- name: Operators
+  href: operators.md
 - name: Videos
   href: videos.md