docs(BA-2545): Add Artifact API descriptions

[jopemachine]

resolves #6083 (BA-2545)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue

---

📚 Documentation preview 📚: https://sorna--6104.org.readthedocs.build/en/6104/

---

📚 Documentation preview 📚: https://sorna-ko--6104.org.readthedocs.build/ko/6104/

[Copilot]

Pull Request Overview

This PR adds comprehensive descriptions to the Artifact API GraphQL schema to improve documentation and developer experience. The documentation provides detailed explanations of artifact workflows, status transitions, and operation purposes.

• Replaces basic "Added in X.X.X" descriptions with detailed multi-line documentation for all artifact-related GraphQL types
• Documents the complete artifact workflow: Scan → Import → Use, including status transitions (SCANNED → PULLING → PULLED → AVAILABLE)
• Adds explanations for key concepts like artifact types, availability states, and admin-only operations

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/ai/backend/manager/api/gql/artifact.py	Adds comprehensive descriptions to all artifact-related GraphQL input/output types and operations using dedent_strip utility
docs/manager/graphql-reference/v2-schema.graphql	Updates generated GraphQL schema documentation with new detailed descriptions
docs/manager/graphql-reference/supergraph.graphql	Updates supergraph schema documentation with new detailed descriptions

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

This input class DelegateeTarget is missing the detailed description treatment that other classes received. It should use dedent_strip with a comprehensive description explaining its purpose for delegated scanning operations.

Suggested change

	@strawberry.input(description="Added in 25.15.0")
	@strawberry.input(
	description=dedent_strip("""
	Added in 25.15.0.
	Specifies the target for delegated scanning operations, including the delegatee reservoir registry
	and its associated remote registry. Used to direct scan requests from a delegator to a specific
	delegatee's registry, enabling discovery and registration of artifacts from external sources
	via delegation.
	""")
	)

[Copilot]

There's an unnecessary comma after "and" that creates a grammatically incorrect sentence fragment. It should read "Artifacts are discovered through scanning external registries and can have multiple revisions."

Suggested change

	Artifacts are discovered through scanning external registries and,
	can have multiple revisions.
	Artifacts are discovered through scanning external registries and can have multiple revisions.