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.