The NIXL integration commit (1b8f467) broke DOCA-only builds in two
ways:

1. internode_doca.cuh lost the torch / pybind11 / standard headers that
   the original internode.cuh provided. Building with HYBRID_EP_MULTINODE=1
   and USE_NIXL=0 (the documented DOCA path) failed with 9 nvcc errors on
   the torch::Tensor / py::list / memcpy expressions inside
   RDMACoordinator::exchange_remote_rdma_info.

2. A committed empty .use_nixl marker file plus a setup.py fallback
   forced use_nixl=True on every build, regardless of the USE_NIXL env
   var. This silently disabled the DOCA path even when the user
   explicitly opted out.

This change:

- Restores the missing system / torch / pybind11 includes in
  internode_doca.cuh (relocated from the original internode.cuh; guarded
  by #ifndef USE_NIXL so they cannot leak into the NIXL build).
- Removes the .use_nixl marker file and the corresponding fallback
  branch in setup.py, making USE_NIXL=0 the actual default again.
- Restores the DOCA branch in setup.py to the pre-NIXL behavior:
  subprocess.run(make src.build, ..., check=True) with no NIXL-suggesting
  SystemExit message.
- Updates docs/README_Hybrid-EP.md to drop the .use_nixl workaround and
  point users at pip install --no-build-isolation when env vars are
  stripped by build isolation.

DOCA build configuration (sources, includes, libs, extra_objects, link
args, NVCC flags) is now byte-for-byte identical to the pre-1b8f467
state. NIXL paths are unchanged and remain opt-in via USE_NIXL=1.

Fixes deepseek-ai#606