docs: system-image.md — the boot capsule documented in full

system.img was only mentioned in passing (efi.md, system-requirements.md);
its format, builder, fallback chain, and kernel-side life were spread across
pack-system-image.py, build.zig, efi.zig, initial-ramdisk.zig, and vfs.zig.
New dedicated page covers all of it, cross-linked from every prior mention
and added to the docs index.

docs/system-requirements.md

@@ -117,7 +117,8 @@ hypervisor configured for UEFI firmware and an xHCI USB controller.
   FADT (power / PM timer). Optionally consumed: HPET, DMAR, SPCR.
   (`system/devices/acpi.zig:3`)
 - The loader reads `/system/kernel` off the FAT boot volume, then loads user
-  space: a prebuilt `boot\system.img` capsule when present, otherwise it walks
+  space: a prebuilt `boot\system.img` capsule
+  ([system-image.md](system-image.md)) when present, otherwise it walks
   the volume's `/system` tree (init included) into the initial ramdisk. The
   kernel can boot "kernel-only" without either. (`efi.zig:16`, `efi.zig:68`)