Update agent install docs

[john-david3]

Proposed changes

• Added detail on how to check if agent is installed
• Changed sudo systemctl stop nginx-agent to sudo systemctl restart nginx-agent as described in the text above
• Added a link to setting up NGINX One Console after agent has been installed

Checklist

Before merging a pull request, run through this checklist and mark each as complete.

• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have rebased my branch onto main
• I have ensured my PR is targeting the main branch and pulling from my branch from my own fork
• I have ensured that the commit messages adhere to Conventional Commits
• I have ensured that documentation content adheres to the style guide
• If the change involves potentially sensitive changes¹, I have assessed the possible impact
• If applicable, I have added tests that prove my fix is effective or that my feature works
• I have ensured that existing tests pass after adding my changes
• If applicable, I have updated README.md and CHANGELOG.md

Footnotes

1. Potentially sensitive changes include anything involving code, personally identify information (PII), live URLs or significant amounts of new or revised documentation. Please refer to our style guide for guidance about placeholder content. ↩

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[john-david3]

I have hereby read the F5 CLA and agree to its terms

[github-actions]

✅ Deploy Preview will be available once build job completes!

Name	Link
😎 Deploy Preview	https://frontdoor-test-docs.nginx.com/previews/docs/559/

---

[nginx-seanmoloney]

can we change this to, check the agent log for

"Agent connected"

[nginx-seanmoloney]

sudo cat /var/log/nginx-agent/agent.log | grep "Agent connected"

[nginx-seanmoloney]

Please add shell, see example above