[LEBV-9] Add accordion navigation
🎯 Summary
This MR adds the general accordion navigation confirmed by Lebara. This includes:
- Complete design.
- Internationalisation of title and headers.
- Fully unit tested.
Also fixes:
- Now the header is sticky.
🧪 How to test
The navigation accordion can be tested following the currently implemented navigation flow (not final!):
- In the first step, complete the fields with any number.
- In the second step, just scroll down and click on "Continue".
- In the third step, click in the first option.
- That's all.
🧠 Considerations
- For now only English texts are provided.
- The content of each step is provisional.
🧐 Checklist
- Changes work in Google Chrome
- Changes work in Safari
- UI is responsive