externalSSO: new section about social login no longer working

[ultraschuppi]

Description

https://camunda.slack.com/archives/C076Z98GED8/p1717743102904059?thread_ts=1717737012.745669&cid=C076Z98GED8

this adds a poorly written note about social login no longer working when externalSSO is configured in C8-SaaS.

we have had a couple of occurances of people failing on that, and since we cannot really add a note about that in the login-screen itself the support folks have been asking for a public doc coverage about that so they can add that to the onboarding procedure

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/).

• 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.