Add comprehensive documentation for the Vhost framework, which serves
as the VirtIO backend implementation in NuttX for cross-core communication.
The documentation includes:
- Overview and comparison with Linux Vhost and QEMU VirtIO Device
- Three-layer architecture (Service, Framework, Transport)
- Driver/Device registration and matching workflow
- Source code directory structure
- Complete API reference (data structures, core APIs, macro APIs)
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>