FR: Support showing description in a UI tooltip rather than inline in value.schema (see #932)

See #932 for context. This is suggestion 2.

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.

Move the longer description into a tooltip (a small icon that would display the text either on click or hover in a mini-popup) rather than a smaller font subtitle. Short description could be kept inline, as they are now. The display mode could also be controlled in `values.schema.json`:
```json
"x-onyxia": {
"descriptionStyle": "tooltip/inline"
}
```