Fixes #10082: document .navbar-form

* Adds new section to navbar docs with example
* Updates example classes for all collapsing navbars so their numbers
increment as you scroll

_includes/nav-components.html

@@ -61,6 +61,7 @@
   <a href="#navbar">Navbar</a>
   <ul class="nav">
     <li><a href="#navbar-default">Default navbar</a></li>
+    <li><a href="#navbar-forms">Forms</a></li>
     <li><a href="#navbar-buttons">Buttons</a></li>
     <li><a href="#navbar-text">Text</a></li>
     <li><a href="#navbar-links">Non-nav links</a></li>