GP: link network tools pages

[MartinKolarik]

We link the top-level pages in the footer so we can go from there and always list locations one level deeper on the page. E.g., on the continent page, list all countries; on the countries page, list all cities. This could be a new section above the "What is traceroute?" content and could reuse one of the existing network pages designs.

[jimaek]

At what point do we show the networks? At country level? At city level? At country it would probably be a big list, but only for popular countries

[MartinKolarik]

I'm fine with both but we can do city to keep it simple: continent -> country -> city -> network