docs: refresh home page

[theSuess]

This PR updates our documentation site to newer hugo & docsy versions!
It also:

• remove symlinks in docs as they are not supported by hugo
• updates the about page to reflect the fact that this is now maintained by Grafana labs
• adds search support (fully offline)
• adds light/dark mode capabilities

Screenshots:

[weisdd]

I'm not sure if I like the new header colour, maybe I just got too used to the old one.
For the rest, changes look great, thanks for taking care of the docs :)

Btw, I've pushed a commit to your branch to fix PR validation for hugo. - Hugo version had to be updated not only in hugo.yaml, but also in pr-hugo.yaml. We should probably merge the workflows in future.