docs: add minimal NDS structure example for Issue #17

[SamihaAlii]

This PR adds a minimal example of the Neuroscience Data Structure (NDS) to help users organize subject and session data according to the standard.

The example includes:

• Subject metadata (ID, species, age, sex)
• Session metadata (ID, task, acquisition device, channels/frame rate)

This minimal example can serve as a template for new datasets and improve human-readability for the standard.

Closes #17