[website] installation doc blues
Switching docs to a different branch seems to be broken from at least one doc, e.g. going from
https://huggingface.co/transformers/master/installation.html#caching-models
upper left corner - if you want to switch to a different branch it sends you to a 404 page on all of them:

it has an issue with the base url, note how it links to: https://huggingface.co/transformers/master/master
master twice, so now it prefixes all branches with master/branch

ok narrowed it down - it happens specifically on the installation page: https://huggingface.co/transformers/master/installation.html
I tried a bunch of other pages and it seems to be ok there.

oddly I don't see anything unusual about installation.md in toctree or its content.

if you look at the snapshot - instead of showing `master` for the currently selected version it shows a chunk of the url instead.

![snapshot_8](https://user-images.githubusercontent.com/10676103/110193531-41d0ff80-7de9-11eb-9697-293daa527983.png)

@sgugger 