Footer only available on index page

We added a footer to the docs but it's not displayed when sidebar navigation is visible. It's by design, see the docs. 

> Note that footer will not be displayed when the [SideBar](https://vitepress.dev/reference/default-theme-sidebar) is visible.

https://vitepress.dev/reference/default-theme-footer#footer

@DerZade menitoned that we can solve this via a custom theme.

