Pythontemplatepackage

add rename.py script

**Documentation & Tests**
- test examples from documentation when building it
- test examples from gallery as part of unittests

**GitHub Actions**
- rename GH action files for documentation

**Documentation**
- be nitpicky about brocken links etc
- add Sphinx Gallery example
- add reference to Sphinx Gallery
- add matplotlib requirement needed for Sphinx Gallery

**GitHub Actions**
- test building doc from dev branch
- switch to codecov action v3
- don't trigger actions on main branch by pull requests

First fully functional version