docs: Connect NGINX Agent to NGINX One using an explicit forward proxy

[JTorreG]

Proposed changes

New doc explaining how to use an explicit forward proxy with Agent

Also:

• Removed "reporting" from the Agent\Configure Instances

NMS-46504

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[mjang]

I've added some suggestions. I gather we may replace this entire PR with a link to Squid docs.

[ADubhlaoich]

Generally LGTM, a handful of small LOGAF nitpicks.

[mjang]

Changes needed. Approving to expedite merging.

[mjang]

General question. I know port 3128 is used by Squid. Do all other (major) proxy services use the same port? (When I search through /etc/services, I see ports dedicated to other proxy services.)

So I think we should replace 3128 in the whole doc page with something generic like: <proxy port>

[mjang]

I do think we need to genericize the port number. Comment in place.

But since this is scheduled for release today, approving to expedite