Fixes link to structured typesetting version of the HTML pages

[tirix]

This should fix issue tirix/metamath-web#24.

[GinoGiotto]

Since the old structured pages used to occupy 12GB of data we can live without them for now. Some time in the future I hope metamath/metamath-exe#9 will be merged, even though I acknowledge that solving merge conflicts there looks like a nightmare.

[digama0]

I'd rather not add more web site generation stuff to metamath-exe; I'm really close to being able to replace metamath-exe with mm-web-rs for the main theorem page generation, pending a fix for metamath/metamath-knife#141, which will result in a 400x performance improvement.

[tirix]

Oh I was not aware that metamath/metamath-knife#141 was blocking that project, I should have another look/another try at it.

But your objection is about metamath/metamath-exe#9, not this PR, right?

[digama0]

But your objection is about metamath/metamath-exe#9, not this PR, right?

Right. More specifically, I would like to make that PR obsolete before it even lands, although I am not objecting to the PR contents itself