If you are making movies for the web you don't need iDVD at all. You would only have to 'reverse engineer' it afterwards.
Make the movie in iMovie, adding chapters etc, then export it in a version suitable for the web.
This is why Apple decided to withdraw iDVD and make everyone use the Cloud, like it or not:
Why is there no iDVD on my new Mac?
1. You can't do that on the web. Only on DVDs using iDVD.
2. Look under the settings in iMovie menu for Export.
iMovie 11 tutorials:
and also this:
Break up the slideshow into individual chapters using Quicktime Player 7 and Quicktime Pro. Or in iMovie break it up into individual clips and export. Then add the individual clips to and iWeb site so they can be played independantly from one another.
Post over in the iWeb forum to get more detailed suggestiions.