Allow custom html in body

[ajh123]

We can already define custom head html like

(doc
  ...

  (site
    ...

    (head doc/head.html)
  )

You could insert custom html (from a file like the head is) at the start of the body to include a custom navigation. To "top it off" you could allow people to define custom html at the bottom of the body. This could allow them to define a custom footer.

This would make illuaminate more easily integrate-able into existing sites.

For example this could be used by CC: Tweaked to include a link to the GitHub on a custom navbar across the top, or you might have multiple documentation sites then you probably want a way to link them together.