Improve batch documentation: Content-ID references, $$ syntax, and atomicityGroup (changeset) support

[kbarbounakis]

Is your feature request related to a problem? Please describe.

While the batch feature in OData v4 in themost-framework/express has evolved to support Content-ID ($id) referencing, there is significant interest and need for:

• Robust and clear documentation explaining standard batch Content-ID referencing (via $id in URLs)
• An extension supporting value replacement from previous requests using a double-dollar syntax ($$id.property) within request bodies
• Explicit documentation for atomicityGroup (changeset) support for transactional (all-or-nothing) batch groups
• Best practices for dependency handling, error handling (including 424 Failed Dependency), advanced usage (arrays, nested objects), and migration from single operations to batch + transactions

These features offer complex batch workflows, but developers lack a single, authoritative reference for implementation, examples, and limitations.

Describe the solution you'd like

Develop a comprehensive batch documentation covering:

• OData v4 Content-ID referencing ($id in URLs; using Location/@odata.id from previous results)
• Double-dollar value reference extension ($$id.property for referencing body values of previous requests, including nested/object/array access)
• Use and meaning of atomicityGroup for grouping requests as an atomic transaction (changeset)
• Table of contents, overview, basic and advanced examples, error/roll-back handling, troubleshooting, best practices, API reference, configuration, and FAQ
• Guidance on migration from classic or non-batch patterns
• Practical real-world scenarios (e.g., order workflows, bulk import, financial transfer, tenant migration)

Describe alternatives you've considered

• Learning by trial and error (slow, error-prone)
• Reading OData spec and piecing together scattered information
• Reaching out to maintainers for clarification

Additional context

• An example documentation draft has been prepared (see conversation in this request for content and structure)
• See also: OData v4 Batch Protocol, docs, changesets, Content-ID, and best practices from large OData APIs
• Potential for this documentation to serve as a reference standard for future contributors
• Should include diagrams/tables when possible for clarity
• Documentation should be accessible from the README and as a standalone detailed page/file