Provide documentation as GitHub Page (proper)

[mbfm]

Follow-up story to #89.

Tasks:

• [ ] Move code from https://github.com/eclipse-opendut/eclipse-opendut.github.io (excluding the book/ folder) into this repo into folder opendut-homepage.
• [ ] Create cargo ci doc homepage build command, which builds the book first and then places everything into target/ci/homepage.
• [ ] Add GitHub workflow which calls cargo ci doc homepage build and pushes artifact to eclipse-opendut.github.io repo.

Preparation for #126.