Co-authored-by: C.A.P. Linssen <charl@turingbirds.com>

Reomove Travis CI integration.

* remove Travis yaml and build scripts
* remove Travis badges from README and RTD docs
* replace Travis badge with GitHub Actions badge
* add a RTD badge to the README

Fixes #1338.

Update the PR licensing rules in the PR contribution guide.

* Move PR message instructions to 'Making a PR' section.
* Move coding instructions to 'Write code' section.

* Remove mention of the Copyright Transfer Agreement

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