[DOC]: CPP/OMP/TBB backend execution policies missing from new documentation

While it seems to be still possible to use e.g. `thrust::tbb::par`, I can't find any mention of these execution policies in the new or old documentation, even though the files like e.g. `thrust/system/tbb/execution_policy.h` do encompass doxygen comments. The old github wiki pages like https://github.com/thrust/thrust/wiki/Host-Backends have vanished as well.
Instead users will only find how to change host and device backends via CMake or compiler flags.

Is the use of these execution policies deprecated or discouraged? Are we waiting for someone to write better documentation? Is it just a bug in the documantation that the execution_policy.h pages don't appear? I couldn't find another issue or PR discussing this.