Fix scroll-spy TOC menu to work

[perlun]

There are two problems here:

• The links do not work; you cannot navigate on the page using them.
• The scroll-spy doesn't work either, so the TOC menu does not keep track of where on the page you have scrolled.

[perlun]

Some observations: git bisect led me to realize that 2e093a4 is the "bad" commit, that broke this. But it only broke the spying part. There are actually two different problems at hand here:

• The scroll-spying doesn't work (broken in 2e093a4, which upgraded jekyll and kramdown).
• Clicking on links in the menu don't work. I ran a separate git bisect session for this now and this is caused by 0c76834, where we changed from kramdown to redcarpet as the Markdown rendering engine. This is likely what caused the Markdown rendering to no longer generate ids => link clicking has failed.

We should google "toc with redcarpet" first, to see how to resolve this. Perhaps this fixes the spying part as well, if we are lucky.