docs: document reverse proxy config for job streams

[npepinpe]

Description

This PR documents the proper configuration for long living job streams if there is a reverse proxy or load balancer between the user's clients and the Camunda gateway.

closes #3903

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).
• I included my new page in the sidebar file(s).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[github-actions]

👋 🤖 ✅ Looks like the changes were ported across versions, nice job! 🎉

You can read more about the versioning within our docs in our documentation guidelines.

[lenaschoenburg]

🔧 I think this needs to added to the versioned docs too, to 8.4 at least I think.

[conceptualshark]

@npepinpe I pushed a few updates, but I think this is in good shape otherwise. I assume it should be backported to previous versions as well?

[npepinpe]

Looks good 👍 Hm, there's no job streaming before 8.4, so we could backport to 8.5 and 8.4.

[conceptualshark]

Went ahead and backported to the requested versions - all looks good on my end, so approving here unless there's any other engineering reviews waiting.