Issue description

Currently, you cannot set the rel="..." property on menu items.
This is kind of a feature, but I consider it closer to a bug.

The hugo documentation offers the following example:

[menus]
  # ...
  [[menus.main]]
    name = 'Hugo'
    pre = '<i class="fa fa-heart"></i>'
    url = 'https://gohugo.io/'
    weight = 30
    [menus.main.params]
      rel = 'external'

Theme version

v2.12.2

Hugo version

hugo v0.151.0+extended+withdeploy linux/amd64

Which browser rendering engines are you seeing the problem on?

No response

URL to sample repository or website

No response

Hugo output or build error messages