Expressing Parallelism chapter design

After investigating Specification chapters related to Expressing parallelism we want to propose and discuss the new structure of related topics in the Reference guide.

Currently the Reference guide "Expressing parallelism" part looks like this:

In the Specification all information for this chapter is provided in sections [4.9](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:expr-parall-thro), [4.11](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:interfaces.bundles), [4.12](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_defining_kernels).

We want to propose this structure of Sections:

- Defining kernels
- Range Classes:
- sycl::range
- sycl::nd_range
- sycl::id (could also relate to Item Classes or Range Classes)
- Item Classes:
- sycl::item
- sycl::nd_item
- sycl::h_item
- sycl::group
- sycl::sub_group
- sycl::handler
- Specialization Constants
- Kernel bundles
- sycl::kernel
- sycl::device_image
- Reduction variables

Could you please also review this structure and comment on how we can improve it?

Also, which coverage of those topics do you expect in this part? Some of the sections, f.e. "Kernel bundles", contain a lot of information, which could be overwhelming for the Reference guide. Should we in such cases add less information and just add references to the Specs?

Review the current Reference guide’s “Expressing parallelism” chapter and the related Specification sections 4.9, 4.11, and 4.12. Compare the proposed section structure and determine the expected coverage for broad topics such as Kernel bundles. Done means documenting an agreed chapter organization and guidance on when to summarize or reference the Specification.

Written by the indexing model from the issue text.