Remove package namespace and rename to `sphinx-hep-pdgref`

[Remco de Boer]

In #29, it seems that the sphinxcontrib.hep namespace is causing problems. Generally, Python namespace packages are to be avoided, to it would be better to rename the package to sphinx-hep-pdgref.

Tasks

Beta Give feedback

1. BREAK: deprecate sphinxcontrib-hep-pdgref package sphinxcontrib-hep-pdgref#32
   ⚠️ Interface +1
   
2. Create a PyPI release for sphinxcontrib-hep-pdgref with a deprecation warning and a release note that links to sphinx-hep-pdgref
3. BREAK: rename package to sphinx-hep-pdgref sphinxcontrib-hep-pdgref#31
   ⚠️ Interface +1
   
4. Rename repository to sphinx-hep-pdgref
5. Create a new PyPI release for sphinx-hep-pdgref

[Remco de Boer]

Published v0.2.0 to PyPI
https://pypi.org/project/sphinx-hep-pdgref/0.2.0