[Nix Docs] Add section on using alternate shells

Summary:
Many developers would prefer to use their existing fish or zsh setups.
Detail how to use an alternate shell using 'nix develop'.

Test Plan:

# start zsh shell
zsh

nix develop -c $SHELL
echo $0 # should still be using zsh

Reviewers: ashoat, varun, marcin, inka

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6237