Adds favicon to Dash/Zeal docset, adds OpenSearch metadata. (#25178)

This sets the favicon for the offline Dash/Zeal docs.

Also, sets up the OpenSearch Description metadata file so that people can create custom search shortcuts for the API docs site.

Fixes #6412

dev/docs/lib/opensearch.xml

@@ -0,0 +1,14 @@
+<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/" xmlns:moz="http://www.mozilla.org/2006/browser/search/">
+<ShortName>Flutter API</ShortName>
+<Description>
+Welcome to the Flutter API reference documentation search.
+
+Flutter is Google’s mobile UI framework for crafting high-quality native interfaces on iOS and Android in record time.
+Flutter works with existing code, is used by developers and organizations around the world, and is free and open source.
+
+The API reference herein covers all libraries that are exported by the Flutter SDK.
+</Description>
+<InputEncoding>UTF-8</InputEncoding>
+<Image width="32" height="32" type="image/x-icon">/flutter/static-assets/favicon.png</Image>
+<Url type="text/html" method="get" template="{SITE_URL}?search={searchTerms}"/>
+</OpenSearchDescription>

dev/docs/opensearch.html

@@ -0,0 +1 @@
+<link rel="search" type="application/opensearchdescription+xml" title="Flutter API" href="/opensearch.xml"/>