- Generate timestamped VERSION once, to properly group on Test.PyPI. Example:
  - https://github.com/arbor-sim/arbor/actions/runs/2696804929
  - https://test.pypi.org/project/arbor/0.7rc20220719101015/
- Update/simplify release documentation.
- Update to v0.7

* Update URLs to arbor-sim.org
* update badge links

Set up contribution guides.

Add a contribution guide, as well as 
* Added contribution section.
* Moved content here from the wiki regarding (contribution) processes.

Also added some incomplete / placeholder sections that can be improved later.
* How to write an example
* How to add a test
* Coding style guidelines

Added a CONTRIBUTION.md page for the project.

Contains a link to the wiki with more detailed description of the contribution workflow.