message: docs(elements): add intro connecting angular elements to custom elements
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diff: # Angular Elements Overview
  +Angular Elements are Angular components packaged as custom elements, a web standard for defining new html elements in a framework-agnostic way.
  +
  [Custom elements](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements) are a Web Platform feature currently supported by Chrome, Opera, and Safari, and available in other browsers through polyfills (see [Browser Support](#browser-support)).
  A custom element extends HTML by allowing you to define a tag whose content is created and controlled by JavaScript code.
  The browser maintains a `CustomElementRegistry` of defined custom elements (also called Web Components), which maps an instantiable JavaScript class to an HTML tag.
  