Remove, update, or clarify lectures.html

[wenzeslaus]

The file lectures.html is not up to date because lectures are now linked from topics. Here are the options:

1. Remove the file altogether. (Bad for old links from somewhere.)
2. Keep the current file as is as legacy and put there line saying it is legacy. (Bad for migrations, link fixes, etc.)
3. Keep the file up to date, e.g. using a script. (Requires some work.)
4. Redirect this file to index of topics.