docs(tutorials): add SaaS federation v2 tutorial (#82)
Pairs molecule-core#1700.
- New content/docs/tutorials/saas-federation.md: clean standalone tutorial
on multi-tenant org onboarding, workspace provisioning, fleet inspection,
quota controls, and suspension/teardown. Corrects HTTP 402→409 for quota
gates (RFC 9110: resource-state conflict, not payment failure).
- api-reference.mdx: PUT /workspaces/:id/files/*path now documents the
EC2-Instance-Connect SSH-backed write path for SaaS (non-Docker) workspaces.
- changelog.mdx: 2026-04-23 entry updated with #1700 (new feature),
#1702 (fix), and corrects earlier entry structure.
Co-authored-by: Molecule AI Documentation Specialist <documentation-specialist@agents.moleculesai.app>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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