Drop IE11 support.

Microsoft is about to do the same thing: https://docs.microsoft.com/en-us/lifecycle/products/internet-explorer-11

Plus, Microsoft already has "IE Mode" in Edge which it will support until 2029: https://www.microsoft.com/en-us/edge/business/ie-mode

There will be no reason to keep IE support.

Author: trusktr

README.md

@@ -43,7 +43,6 @@
 - Smart full-text search plugin
 - Multiple themes
 - Useful plugin API
-- Compatible with IE11
 - Experimental SSR support ([example](https://github.com/docsifyjs/docsify-ssr-demo))
 - Support embedded files