add local links to headers

This is super useful for making links to individual sections of the longer how-tos.

I also fixed a small issue with the size of typewriter-text in headers.

The result looks roughly like this:

(notice the # on the right)

on hover it catches a bit of color so that it feels more clicky.

This requires bundling of https://github.com/allejo/jekyll-anchor-headings but I didn't find any better installation possibilities and the licenses seem compatible, so why not. Opinions about better options welcome.

For the testing, I don't know what can get broken (no idea where this theme is applied) but my local copy of howto-cards seems happy with the change.

cc @laurent.heirendt and likely @jacek.lebioda