Add Stack stopgap page (#764)

* stack wip

* add coming soon

* cleanup

* fix src

* fix description

content/components/stack.mdx

@@ -0,0 +1,21 @@
+---
+title: Stack
+status: Experimental
+description: Stack is a layout component that creates responsive horizontal and vertical flows.
+reactId: stack
+---
+
+import ComponentLayout from '~/src/layouts/component-layout'
+export default ComponentLayout
+
+<Note variant="warning">
+ Stack interface guidelines coming soon!
+</Note>
+
+## Anatomy
+
+<img
+  width="960"
+  alt="Example of the Stack component in use showing a text input next to a group of buttons. The stack is shown both horizontally and vertically."
+  src="https://github.com/primer/primitives/assets/18661030/5b8649e0-1c3e-4845-abb1-c339b628d056"
+/>

src/@primer/gatsby-theme-doctocat/nav.yml

@@ -313,6 +313,8 @@
       url: /components/spinner
     - title: Split page layout
       url: /components/split-page-layout
+    - title: Stack
+      url: /components/stack
     - title: State label
       url: /components/state-label
     - title: Subnav