docs: add multi-tenancy administration guide

[RomuDeuxfois]

Summary

Adds a user-facing administration guide for the multi-tenancy feature.

This page lives in docs/administration/multi-tenancy.md and is registered in the mkdocs.yml nav under Administration → Multi-tenancy.

What's covered

Section	Content
Concepts	Tenant definition, platform vs. tenant scope, dual-scope resources
Managing tenants	Create / update / delete (soft-delete 30d) / reactivate — step by step
Users & access	Assigning users via groups, roles per tenant, platform admins
SSO & tenant mapping	openaev.provider.{id}.tenant_id + groups_management parameters with env var equivalents
Integrations	Built-in auto-registration, per-tenant RabbitMQ queues + S3 prefix, connector instances
Configuration reference	All multi-tenancy parameters with env var names and defaults
FAQ	5 common questions (multi-tenant membership, platform resources, deletion, DB isolation, migration)

Sources

All content is sourced from the codebase:

• TenantService.java — lifecycle (create / soft-delete 30d / reactivate / purge)
• TenantInterceptor.java + HibernateFilterTransactionAspect.java — request isolation
• TenantQueueService.java + MinioService.java — per-tenant infra isolation
• BuiltinIntegrationFactory.java + BuiltinTenantRegistrable.java — auto-registration
• application.properties — SSO tenant_id + groups_management params
• TenantApi.java — EE gate (@AccessControl(isEnterpriseEdition = true))

Checklist

• Matches the style of existing admin pages (enterprise.md, users-and-rbac.md)
• Uses MkDocs Material admonitions (tip, warning, danger, info)
• All parameters documented with their environment variable equivalent
• EE requirement clearly called out at the top
• Screenshots to add (Settings → Tenants UI) — left for follow-up