feat: add description attribute for nlu values

[MohamedAliBouhaouala]

Motivation

The following PR adds a description attribute for each NLP value. This will allow us to attach additional explanatory information or context to the NLP value in the system.

//@todo : add support for search functionality on NLP values' descriptions

Fixes # (387)

Type of change:

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
  Screencast from 2025-03-21 12-13-04.webm