FR: Support UI-specific description and title in value.schema (see #932)

See #932 for context. This is suggestion 1.

The `title` and `description` fields from the values schema files are meant for documentation and not user interface definition. As such, they are often defined in a more verbose or redundant way than is preferable in our context.

The `title` and `description` fields may be defined in Onyxia catalogs to better fit the needs of a user interface. However the resulting `values.schema.json` file would then be hard to read or reuse in a different setting. Instead it would be useful to provide alternatives for these (rightfully) descriptive fields. For instance:
```json
{
"title": "A precise field title",
"description": "An exhaustive description",
"x-onyxia": {
"title": "Short title",
"description": "Extra info in subtitle or empty string to hide"
}
}
```