Add Documentation/Comments

[raphael-goetz]

The current documentation is non-existent. To enhance readability and usability in the future, we should add detailed documentation or comments. Additionally, the protos may be difficult for others to understand, as we are currently the only ones who have worked on them. Clear documentation will ensure that the code is accessible to others in the future. @Taucher2003

An example repository how it could look like:
https://github.com/google/automotive-design-compose/tree/main/proto