Re-introduce multi HTML documentation version

Since we recently updated doc generating process, there are only single-HTML and PDF docs now. However, as Martin correctly pointed out, this will break any links out there that have been pointing to latest doc version in the multi HTML variation.

Note that the latest 3.1 release still had multi HTML present (3.1.5.SP1) whereas latest 4.x did not (4.0.Beta1). Therefore, this really only affect 4.x.

If we choose, we could even retrospectively generate the doc for 4.0.Beta1 to fix that (which I will probably do).

We can also choose if we want to re-add links to multi HTML output to Weld website, or if we want to just keep it as a fallback for existing links. ATM there are only links to PDF and single-HTML.