docs: add TunaOS QEMU/KVM evaluation guide

[hanthor]

What changed

• Add docs/tunaos/evaluating-in-a-vm.md, a quick-start guide for safely evaluating TunaOS in a QEMU/KVM VM.
• Cover ISO selection and checksum verification, VM sizing, NAT networking, virt-manager setup, and direct QEMU CLI usage.
• Document qcow2 snapshots, first-boot expectations, cleanup, and the scripts/build-qcow2.sh source-build path.
• Add the guide to the TunaOS Developer Guide sidebar.

Why

Give evaluators and new contributors a reversible 15–20 minute path to try TunaOS without repartitioning or changing their host installation.

Validation

• git diff --check
• npm run build (passes; existing repository warnings remain for duplicate XFCE routes, deprecated config, and unrelated broken links)

Fixes #216