Add missing upstream remote command to quickstart guide

[sarthyparty]

Proposed changes

Problem: The Quickstart instructions were missing the command to the upstream remote

Solution: I added the missing command to add the upstream remote

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

[kate-osborn]

🚀

[pleshakov]

@sarthyparty
a few style nits related to https://github.com/nginxinc/nginx-gateway-fabric/blob/main/docs/developer/pull-request.md#commit-message-format and commit message

(1) let's expand commit message with problem/solution statements as described in the document above.

(2) better to use active voice in commit messages. For example, Fix git clone command in developer guide. Same for PR name.