Add link to xref.html in the format gallery 
While digging in the generating code for the [format gallery](https://formats.kaitai.io/), I noticed that every time it generates page https://formats.kaitai.io/xref.html, which is quite a neat summary of all formats included with their licenses and cross-references. It's a pity that there isn't any link leading to it, so nobody can actually get there. So I think it makes sense to add one. Probably on the [format gallery homepage](https://formats.kaitai.io/).