Allow for footer items which contain all three of `text`, `icon`, and `href` subkeys (nav-style)

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/10288

^{Originally posted by connortwiegand July 13, 2024}

Description

Hi,

In navbars and sidebar contents, you can pass an item of the form

- icon: foo
  text: bar 
  href: baz.biz

It would be nice if this was supported in the footer as well. Currently, the page-footer key under website allows for

- icon: foo
  href: baz.biz

or

- text: foo
  href: baz.biz

But not all three. A current workaround is to use, e.g.,

- text: "{{< fa rss >}} Feed"
  href: "index.xml"

I think that unification to the text/icon/href schema would be clear + useful, if it's not too much trouble to implement.

Thank you!