Document use of `uv venv --system-site-packages` to support Swabian TimeTagger

[Benjamin-Nussbaum]

The Swabian Instruments TimeTagger library has been updated to version 2.19 and seems to no longer require numpy versions <2.0. When updating the dependency, we can also remove the redundant uv.sources entry in pyproject.toml and add a note that in order to make uv aware of the library, it is necessary to run uv venv --system-site-packages.