Document nada new-program command

[navasvarela]

Feature Description
This PR https://github.com/NillionNetwork/nillion/pull/2286 implements a new command for nada to create new programs, reducing the need for users to manipulate nada-project.toml.

This command:

• Creates a new template file with the required name under the project source folder
• Updates nada-project.toml, adding a new entry for the program.

Why is this feature important?

• It provides good developer experience.
• It simplifies the process of creating new programs
• It reduces manual errors when editign nada-project.toml

How do developers use the feature?
If a developer wants to create a new program called amazing_program, she can do this:

nada new-program amazing_program

This will create a file src/amazing_program.py and update nada-project.toml.

Usage or commands
See section above.

Additional context
https://github.com/NillionNetwork/nillion/pull/2286