fix navigation buttons in footer

[SamGuay]

The footer in mkdoc was updated so the footer in portray is not up to date. If you look at https://timothycrosley.github.io/portray/, we can clearly see that there is something wrong at the bottom.

This PR makes the navigation buttons in the footer look as seen on mkdoc doc.

Note that I updated the footer but I left out the new copyright partial as portray uses its own copyright mechanism.