ci: Make adding area:docs label trigger more precise

[Yaminyam]

Main idea

backend.ai/.github/labeler.yml

Line 10 in 7ac278f

- 'src/ai/backend/manager/api/**/*.py'

Currently, 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