Make all Actions that Require Setup Opt-in

[carschno]

When a new package is created from the template and pushed to Github, the developer receives a series of emails about failed actions, e.g. for Sonarcloud (see e.g. #282), the link checker (e.g. #355) etc. This is overwhelming and causes a lot of friction, hence frustration and distraction.

This issue is not about fixing the specific issues, but about reversing the perspective: tasks like adding Sonarcloud checks, generating readthedocs should not default to 'on' for a new project (opt-out). They should be easy to activate once the project reaches an appropriate stage (opt-in). Such a paradigm would suit the common development pattern of new (research) software packages better, that start with 'hacky' development, and slowly converge towards a releasable package.

To summarize:

1. [ ] All workflows should pass when a newly created project is pushed to Github
2. [ ] Workflows that require extra setup (readthedocs, Sonarcloud etc.) should be deactivated
3. [ ] There should be instructions (Github issues) that describe the steps on how to activate these workflows

[carschno]

This should also contribute to #345 .

[egpbos]

I support this. Feel free to make a PR!

[fdiblen]

We made a major refactoring. Users can now select the features and workflows themselves.

If a user opts for features requiring extra steps, related GitHub Action will fail. This is unavoidable because these external services need to be configured.

[fdiblen]

Solved by #551. Please reopen if needed.