Add section on semantic versioning to the development section in the docs

[nbro]

We should provide some guidelines on how to increase the versions after having made changes. We can also link to some site that describes semantic versioning more in detail.

TODO

Explain how to increment the versions when

• some breaking change is introduced
• a new feature is introduced but no breaking change
• a bug is fixed (with or not breaking changes)

[nbro]

There's already a link there to semvar