Tezt-Cloud: Add documentation for scenarios

What

Documentation updates for tezt-cloud scenarios:

• LAYER1 : revamping
• DAL: addition

Why

For people to better understand what we do with the current scenarios, and have correct and up-to-date commands to run experiments.

How

Manually testing the MR

1. Check that the documentation makes sense.
2. Test that the commands presented work.
3. Update with your own knowledge in case I missed something important.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR