Allow user to set homepage to a section

[rikhuijzer]

By default, the homepage is not visible in the menu. However, if a user includes useful information on the homepage (which is also added to the offline outputs like PDF via include_homepage_outside_html), the user probably also wants that page to be accessible via a button in the menu.

Basically, this means that the homepage headers needs to be added to the menu and that clicking the button will move the user towards the homepage /. Alternatively, maybe it is easier to add a redirect from the homepage to the first page from contents, which might be slower and therefore not so good.

[rikhuijzer]

This is not really a priority because we need to put the PDF link somewhere anyway.

[rikhuijzer]

This is not really a priority because we need to put the PDF link somewhere anyway.

Could be done via

```{=html}
You can download the PDF _here_.
```

An easy way could be to just copy the first html page to index.html.