docs: add JEP-0014 Virtual Scalable Exporters proposal

Propose a Virtual Scalable Exporter subsystem for Jumpstarter that
manages pools of virtual targets with configurable autoscaling via
per-provider CRDs (QEMUExporterPool, AndroidExporterPool, etc.).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: mangelajo