Docs: document using Dev Proxy in GitHub workflows

[waldekmastykarz]

Document:

• [ ] role of the CI env var in suppressing prompting
• [ ] waiting for Dev Proxy to start
• [ ] stopping Dev Proxy (graceful via kill -INT vs. hard kill -KILL (kill -9))

Integrate with the existing page at https://learn.microsoft.com/en-us/microsoft-cloud/dev/dev-proxy/how-to/use-dev-proxy-with-github-actions

Related resources

• https://github.com/microsoft/dev-proxy/pull/697/files
• https://github.com/waldekmastykarz/proxy-ci-javascript

[waldekmastykarz]

PR submitted waiting for review