Close documentation structure gaps against Arc for .NET

[woksin]

Changed

• The OpenAPI documentation is split into concept, enum, command, query and model-bound topic pages in the Arc for .NET order. The schemas page keeps the worked example and its existing anchors, and the command and query pages describe the validation-only operation, 401 responses and declared-result paging.
• New identity contracts reference: IdentityDetailsProvider registration, the /.cratis/me response and its status codes, and the principal-first trust model compared with the cookie-first identity in Arc for .NET.
• New end-to-end tenancy guide, from tenant selection and membership checks to tenant-scoped MongoDB, Drizzle and Chronicle reads, including how long a tenant decision lasts.
• New SQL page on schema and column mapping, codecs, host-owned drizzle-kit migrations, change ordering and database credentials.
• The contributing guide maps the Arc for .NET test applications to the TypeScript samples, adapter suites and contract checks.
• Paging and sorting argument names are documented as reserved query-string names.

Refs #78