Create a simple index page with OpenSearch metadata

With this metadata browsers can suggest to add mdn.io as a search engine.

For example, here is how Firefox presents it: https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox#w_add-a-search-engine-from-the-address-bar

To add it, simply add in your index page:

<link rel="search"
      type="application/opensearchdescription+xml"
      href="https://mdn.io/opensearch.xml"
      title="MDN Web Docs"
/>

And host /opensearch.xml that should be something like this:

<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/"
                       xmlns:moz="http:/www.mozilla.org/2006/browser/search/"
>
    <ShortName>MDN Web Docs</ShortName>
    <Description>Search MDN Web Docs</Description>
    <InputEncoding>UTF-8</InputEncoding>
    <Image width="16" height="16" type="image/png">
        https://developer.mozilla.org/static/img/favicon32.7f3da72dcea1.png
    </Image>
    <Url type="text/html" method="get" template="http://mdn.io/{searchTerms}" />
    <moz:SearchForm>http://mdn.io</moz:SearchForm>
</OpenSearchDescription>

More info here: https://developer.mozilla.org/en-US/docs/Web/OpenSearch