doc: use a separate `requirements-rtd.txt` for Read the Docs

[bgilbert]

RTD apparently tries to use the Ubuntu libclang.so even though the libclang Python package provides its own copy. A docs build on vanilla Ubuntu 22.04 doesn't, and it's not clear why. Since pinning to an older clang Python package is known to work, use that approach in a separate requirements-rtd.txt.

Install the libclang-dev package in RTD to get the libclang.so symlink, so we don't have to reinstate the search logic in conf.py.

[jcupitt]

Ahh I hadn't thought how easy it was to have a separate requirements for rtd, nice.

[jcupitt]

And the rtd build worked \o/