Create sub-site nav variant

[inesdgomes]

Hi @beatrizmartinmartins As mentioned in this ticket, we think the current design for sub-sites nav may not cover all of our needs. There are quite complex sub-sites that would require a more flexible nav. So what if:

1. Sub-sites with a simple information architecture use the existing design.
2. Sub-sites with a more complex information architecture use a variant of the global nav.

Let's discuss this on Wednesday but I started putting this guidance here: https://brand.ilo.org/document/132#/-/headers