Bad URL in `README`

[ocaisa]

The README references https://eessi.io/docs/no-proxy in some output, but this location does not exist. I tried to find an alternative to make a PR, but didn't see anything in the EESSI docs that was a good fit.

[boegel]

That's my bad, I started working on a docs PR for this, but didn't finish it, I'll get back to it...

The idea is to make https://eessi.io/docs/no-proxy to a (new) page like https://www.eessi.io/docs/filesystem_layer/no-proxy or maybe https://www.eessi.io/docs/filesystem_layer/cvmfs#no-proxy .