I've gone and implemented it anyway (in commit b9920b16), because I
wanted to see what it would be like. It is for @def* commands only
at the moment, not for @anchor or for section headings. Turn it off
with the -c PERMALINKS=0 option (variable name subject to change).
I'll try this out soon. I think having such links is really important for the html version.
I tried this out using the maxima manual. This looks nice and works very well! Not sure I like the fact that I can only see the link when I hover over the item. I didn't check on a mobile device, but if this is how it works on mobile, that's not so good. Presumably, I can change this behavior with a bit of CSS.
I think links for @anchor is important since it's something the author had to say explicitly, so having that available in html seems appropriate. Perhaps I'll change my mind after I see what you've done so far.
Having seen these new links, I'm not sure what to do about anchors. In maxima's manual, the anchors often point to the first entry of a deffoo. So now there would be two links to the same place, with different ids. In this case, it's not terrible. But if there's an @ref to some @anchor, to get the link you'd have to find any @ref pointing the anchor to get the link. That's not so convenient.