See this tutorial: https://markjaquith.wordpress.com/2006/03/04/wp-tutorial-your-first-wp-plugin/
In this tutorial, if you write the text “foo” the plugin converts this word to “bar”.
In your plugin, you can search for your special tag, for example <embedpage page.html> (or any other syntax of your invention) and replace it by the HTML code returned by a PHP call to the page into the tag. You can use fileopen function in PHP to read the code from the website 2.
regards!
