Document methods for customizing the logo

[hmpf]

From #72

Allows for overriding the navbar logo with a custom logo through context variables. A developer may for example create a context processor to change the logo and its associated properties:

def logo(request):
    return {
        "logo": {
            "file": "logo_color.png",
            "alt": "geant-argus",
            "url": "home",
        }
    }