When you have your intro movie ready, just export the file as usual, and add it to an HTML page. Be sure to always add a Skip button so that people who don't have the patience for a movie to load or have already seen the intro can just go straight to the main site. (In this case, you can also use "cookies" to have the browser automatically skip the intro.)
If you have any questions after reading this article, please don't hesitate to contact me! I'll be happy to answer any questions you may have!