Add documents for rosidl::Buffer features (backport #6440)

[mergify]

Description

Introduces documentation for the new rosidl::Buffer feature and its pluggable backend architecture.

Pages created in this pull request to cover the features are::

• Concepts/Intermediate/About-Buffer-Backends.rst — conceptual overview:
  the Buffer<T> / BufferImplBase<T> / BufferBackend split, the
  descriptor round-trip, discovery hooks, and the "base backend vs
  composed backend" pattern (CUDA as a base; PyTorch as a composed
  backend that can layer on top of multiple bases). Also clarifies that
  the plugin interface is RMW-agnostic — get_descriptor_type_support()
  returns a generic aggregate handle, and the RMW (currently
  rmw_fastrtps_cpp) resolves it internally.

• How-To-Guides/Using-Buffer-Backends.rst — user guide for enabling a
  backend on a subscription via
  SubscriptionOptions::acceptable_buffer_backends
  (""/"cpu" / "any" / comma-separated list), C++ and Python
  examples, a per-RMW support matrix, and the three practical rules for a
  compatible pub/sub pair (same backend installed on both sides, same RMW,
  aligned package versions). Also emphasises that intra-process /
  inter-process / inter-host transport scope is a property of each
  backend, not of rosidl::Buffer.

• Tutorials/Advanced/Writing-a-Buffer-Backend.rst — vendor-facing
  step-by-step guide for implementing and packaging a new BufferBackend
  plugin: interface walkthrough, descriptor design (including the 4096-byte
  kMaxBufferDescriptorSize limit), BufferImplBase<T> and
  BufferBackend implementation, pluginlib registration, CMake/package.xml
  scaffolding, user-facing API patterns (allocate_msg, from_buffer,
  to_buffer), the composed-backend pattern, and a ship checklist. Uses a
  generic mydev backend in prose and cross-links the real CUDA, Torch,
  and demo backends for concrete reference.

• Tutorials/Demos/GPU-Buffer-Transport.rst — runnable end-to-end demo
  based on robot_arm_demo from ros2/rosidl_buffer_backends_tutorials,
  comparing CUDA zero-copy vs CPU-serialised transport at several
  resolutions, with the benchmark table reproduced from the demo README.

Other registration / discoverability page updates:

• Concepts/Intermediate.rst, How-To-Guides.rst,
  Tutorials/Advanced.rst, Tutorials/Demos.rst — new pages added to the
  respective toctrees.
• Glossary.rst — new entries for "Buffer", "Buffer backend", "Base
  backend", "Composed backend", "Buffer descriptor", and "Acceptable
  backend list".
• The-ROS2-Project/Features.rst — added a "Pluggable buffer backends"
  row (marked experimental, currently supported in rmw_fastrtps_cpp and, C++ user-facing APIs only).
• Related-Projects.rst — added a "Community rosidl::Buffer backends"
  section linking to ros2/rosidl_buffer_backends and
  ros2/rosidl_buffer_backends_tutorials.

Did you use Generative AI?

Yes. Cursor with Claude Opus 4.7 was used to assist with the draft version of the docs included in this pull request.

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This is an automatic backport of pull request #6440 done by [Mergify](https://mergify.com).

[github-actions]

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