toc active item highlight

[jakubabrzy]

In this PR we changed variable of nav header to improve scroll functionality which highlights item after the click on right sidebar navigation. Previously it calculated it wrong because docusarious thinks that our sticky nav has 24px (even it isn't visible) so either we can set --ifm-navbar-height variable to 24px to improve it or set this property hideOnScroll to true. I have choosen second option as it could have smaller impact on rest of behaviours in the app which base on --ifm-navbar-height

closes #109

[Roma36]

@jakubabrzy I can confirm that it behaves well in most cases, but there's still one case where it feels buggy - when you click on the last item in TOC.
On screenshot, I've clicked the last item but the one before last got highlighted. Maybe it's related to the fact that both items fit on the screen. Could you please take a look at this?
https://6cf67d9d.opentf-website.pages.dev/docs/cli/init/#reinitializing-only-modules

[jakubabrzy]

@Roma36 It looks like another docusarius issue - it is associated with scroll behavioour when we see more than one section at the bottom on the screen. If you go to https://docusaurus.io/docs/search#using-your-own-search and remove footer from html to see more than one section it will be also broken.