feat(desktop-modeler): add variable tab documentation

[marstamm]

Description

This PR adds documentation for the Variables Tab. This is a new feature to be released with the September alpha release cycle.

related to https://github.com/camunda/product-hub/issues/2011

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]

👋 🤖 🤔 Hello! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.5/.

• docs/components/modeler/desktop-modeler/img/variables-tab.png
• docs/components/modeler/desktop-modeler/variables.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.