The alternative for the home link on the logo does not mean anything

This was reported first in the HAN accessibility report.

`A link text needs to explain clearly where the link will
lead to. The XWiki logo does not have an alt text that
explains it’s purpose: it is serving as a link to the
homepage.`

Right now it's just `logo`, which is descriptive enough for the picture, but not for the link nesting it.