Add categories to mark active in menu correctly

[xPaw]

Fixes #156

[astorije]

I would love to not reintroduce the category keyword in the front matter for this really, as it's cumbersome and error-prone.
I was about to do it using URL prefix because really it makes sense, if you're under /docs/guides/foo and you go back to docs/guides you should go to the parent page. Mind doing that instead?

I'm aware that this won't work for the unofficial install methods method and hmm... I'm not too offended by this one not marking the install page as active really. For one thing, it's the only page in there, and... I have suggested to kill it in a couple places 😬

[xPaw]

I tried the URL, but it uses the folder name, so it doesn't match, unsure if there's another method