NGINX Agent: Add v3.0 upgrade guide

[dhurley]

Proposed changes

Add a page describing how to upgrade from NGINX Agent v2 to v3 and also document the rollback procedure.

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[github-actions]

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[dhurley]

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

[github-actions]

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[JTorreG]

Suggested change

	If your NGINX Agent container was apart of a config sync group, then your NGINX Agent config needs to be manually updated to add the config sync group label.
	If your NGINX Agent container was not a member of a config sync group, then your NGINX Agent config must be manually updated to add the config sync group label.

[JTorreG]

@dhurley can you confirm that apart in here means it was outside of config sync groups and not a typo?

[dhurley]

Maybe this is a better way to say it.

If your NGINX Agent container was previously a member of a config sync group, then your NGINX Agent config must be manually updated to add the config sync group label.

[JTorreG]

@dhurley @nginx-seanmoloney , finished my 1st review pass. I think it will still need more refining