CSS styling enhancements

[mesellings]

Description

This PR fixes the outstanding CSS issues from #4324

• Border on API schema tabs.
• Extraneous break tags added into Getting Started guides.

(And also any other enhancements/fixes that may arise following feedback on the recent CSS changes).

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.

[mesellings]

FYI: @pepopowitz, I'll hold this open for a few days to see if there are any immediate changes needed following the styling release, no need to review yet 👍

[github-actions]

The preview environment relating to the commit 73c7bfd has successfully been deployed. You can access it at https://preview.docs.camunda.cloud/pr-4456/index.html

[pepopowitz]

LGTM, thank you!