ci: Make adding area:docs label trigger more precise

### Main idea

https://github.com/lablup/backend.ai/blob/7ac278f6940b24c9872c52ee4385a56d06176935/.github/labeler.yml#L10Currently, a label is added to indicate that the docs file must be changed when API-related files change, but the actual API specifications may not change even if API-related code changes.Exclude these cases and add labels only when the actual API specification has changed.

- Like graphql.schema, add the restapi specification document as spec.json to the repository.
- Modify to add docs label when changing graphql.schema and spec.json.
### Alternative ideas

_No response_

### Anything else?

_No response_

JIRA Issue: BA-124