Add a favicon to the base page

Add a favicon style link from the base webpage to use the PNG image already existing.

This is a tiny cosmetic change that makes it easier to spot mentors among different tabs. E.g. mentors.d.n vs. local favicon version:

Alternatively, a true "favicon.ico" file could be placed at the root, but that needs to be .ICO format, not png.

Closes: #162 (closed)