docs: document the privacy-filter.cpp backend in README and compatibility table
The privacy-filter.cpp backend (#10360) was registered in backend/index.yaml
and referenced from the PII feature docs, but was missing from the backend
catalog surfaces. Add it to the README "Backends built by us" table, the
compatibility table (Utilities & Other, CPU/CUDA 13/Vulkan), and the backend
type list in the backends feature doc.
Assisted-by: Claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>