docs(modal): Add session expiration variant to design guidelines.

[edonehoo]

Add a "session expiration" variant to the modal design guidelines to provide guidance similar to these previous PF3 docs: https://pf3.patternfly.org/v3/pattern-library/communication/session-timeout/#overview

Also crosslink to the login page mention of session timeout alerts.

Add to q1 2026 milestone once that's been created

---

Jira Issue: PF-3276

[maryshak1996]

The Hybrid Cloud Console team would really benefit from this!