nixos/doc: add instructions for installation behind a proxy (#45854)

The instructions to install nixos behind a proxy were not clear. While
one could guess that setting http_proxy variables can get the install
rolling, one could end up with an installed system where the proxy
settings for the nix-daemon are not configured.

This commit updates the documentation with

1. steps to install behind a proxy

2. configure the global proxy settings so that nix-daemon can access
internet.

3. Pointers to use nesting.clone in case one has to use different proxy
settings on different networks.