examples: add math example (#1461)

* examples: add math example

* docs: add math example

docs/examples/math/index.ts

@@ -0,0 +1,7 @@
+export { default as MathApp } from './App.vue?raw'
+export { default as MathElements } from './initial-elements.js?raw'
+export { default as MathValueNode } from './ValueNode.vue?raw'
+export { default as MathOperatorNode } from './OperatorNode.vue?raw'
+export { default as MathResultNode } from './ResultNode.vue?raw'
+export { default as MathIcon } from './Icon.vue?raw'
+export { default as MathCSS } from './style.css?inline'