[docs] Fix nvm setup instructions

Summary:
Fix nvm setup instructions by adding a code snippet Homebrew prints for M1 Macs under the Homebrew instructions portion of the nvm setup.

Related Linear issue with full context here.

The current code snippet on the dev environment setup is

export NVM_DIR="$HOME/.nvm"
[ -s "/usr/local/opt/nvm/nvm.sh" ] && . "/usr/local/opt/nvm/nvm.sh" --no-use # This loads nvm
[ -s "/usr/local/opt/nvm/etc/bash_completion.d/nvm" ] && . "/usr/local/opt/nvm/etc/bash_completion.d/nvm"  # This loads nvm bash_completion

This code snippet is only printed by Homebrew on Intel Macs, the updated snippet on an M1 Mac looks like:

Updated the docs to acknowledge the differing outputs based on the user's processor, would love to hear feedback regarding the exact verbiage and wording.

Test Plan: Ran the dev environment setup on my M1 Macbook and copied the code snippet Homebrew printed at this step.

Reviewers: varun, ashoat

Reviewed By: varun, ashoat

Subscribers: palys-swm, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3866