REL-542 release ska-pst-0.3.0
Need to release ska-pst-recv-0.7.0 to publish the bug-fixed image in the CAR that is required to successfully release ska-pst-0.3.0.
Following AJ's lead in ska-pst-dsp, I also deleted version.txt and updated docs/src/conf.py and CMakeLists.txt to use .release instead.