Add vue-router docs favicon?

[BARNZ]

What problem does this feature solve?

I notice the vue-router docs does not have any favicon. Can one be added?

What does the proposed API look like?

favicon.ico

Just helps to identify it when users have the tab pinned like me:

[posva]

It would be the Vue icon though

[BARNZ]

Yeah saw that with the vue docs and vuex docs

Could do a slight little variant to distinguish them :)

[posva]

for anybody doing the PR, no variation is needed (vuejs/vue#6305)

[BARNZ]

Added via #3002