Introduce foundation/modules concept

[HenrikFricke]

Right now, we have only defined labs in the workshop. As the workshop grows and we introduce more and more labs, we need to find a more flexible approach. Ideally, we have a thin layer in the beginning and many independent modules based on the foundation. That being said, we'd like to refactor the parts Welcome, Lab 0 - Setup, and Lab 1 -REST API to foundation and all further labs to modules. Foundation and modules should be clearly separated in the sidebar. Modules should not have dependencies on other modules by design.

Acceptance criteria:

• Introduce foundation and modules in the sidebar
• Get rid of numbered labs
• Refactor the names of the code packages
• Rework welcome page to explain the foundation/modules concept

[HenrikFricke]

@bracki @tillkahlbrock Do you have other ideas we should consider? I tried to document our ideas and things we have discussed in the last weeks.

[bracki]

I'd love to see intro stories to the modules that glue everything together narratively.
E.g., "given our Todo list REST API we now want to extend it with automatic reminders, for this we need ... ". Otherwise I see all points addressed.