Check readme examples

The readme examples are partially outdated. For example, it still shows the addContent method.

- This should be fixed and the examples should be checked
- Consider some automated workflow for the examples. The Readme is a bit bloated with examples anyway. We should consider making tests out of them (if they aren't) and link to them instead. This will shorten the readme. We could keep one representational example in the readme so the user gets an immediate idea of how this will work.

Side note: I added a transition guide in the [release notes of v2.0.0](https://github.com/kit-data-manager/ro-crate-java/releases/tag/v2.0.0) regarding the renamed addContent function.

Start by reviewing the README examples against the current Java API, especially the outdated addContent method, and consult the v2.0.0 release notes transition guide. Done means the examples are current; also determine whether the existing examples are covered by tests and whether the README should retain only one representative example.

Written by the indexing model from the issue text.