docs(rest): Add REST Connector proxy documentation

[johnBgood]

Description

Related to #3252

To be merged around the November release.

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 8.6 and are in /versioned_docs directory.
• My changes are for 8.7.0-alpha1 and are in /docs directory (aka /next/).

• 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.

[mesellings]

@johnBgood Does this need to be backported for other versions, or is it ok just for 8.6, next?

[github-actions]

The preview environment relating to the commit d799811 has successfully been deployed. You can access it at https://preview.docs.camunda.cloud/pr-4469/index.html

[mesellings]

@johnBgood Lgtm - I've committed changes directly to the branch for you 👍

[johnBgood]

@johnBgood Does this need to be backported for other versions, or is it ok just for 8.6, next?

@mesellings It's only 8.6+ :)
Thanks for your review!

[mesellings]

@johnBgood Shall I merge and release this for you?

[johnBgood]

@mesellings I'll be released next week, so I don't know when we can merge it. I'll let you decide on it :)

[mesellings]

@johnBgood Do you mean the dev feature this relies on to work is only released next week or would this work currently (now) before the release?

[johnBgood]

@mesellings
this:

Do you mean the dev feature this relies on to work is only released next week

[mesellings]

Thanks @johnBgood I'll hold off and release this on Friday then to avoid people trying to use the proxy feature as a result of the docs 👍