Add docs for installing nginx-agent from repository

[Dean-Coakley]

Proposed changes

• Add install instructions for installing from NGINX OSS repository
• Add install instructions for installing from NGINX Plus repository
• Fix typos
• Move logs and systemd info from install doc to getting-started doc
• Add upgrade doc

Checklist

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

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[JTorreG]

comments + suggestions