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

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[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
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[john-david3]

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

[github-actions]

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