Data access chapter design

We want to propose a new structure for the "Data access" chapter with updated content.

Currently the Reference guide "Data access" part looks like this:

In the Specification all information for this chapter is provided in section [4.7](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:data.access.and.storage) and sycl::private_memory in [4.9.4.2.3](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_parallel_for_hierarchical_invoke).

We want to propose this structure of sections:

- Buffers
- Host allocation
- +Host memory sharing
- Accessors
- General accessor concept (4.7.6 - 4.7.6.8)
- sycl::accessor (4.7.6.9 - 4.7.6, 4.7.6.12)
- sycl::host_accessor (4.7.6.10, 4.7.6.12)
- sycl::local_accessor (4.7.6.11, 4.7.6.12)
- Unsampled image accessors (4.7.6.13)
- Sampled image accessors (4.7.6.14)
- Synchronization primitives
- Address space classes (Multi-pointer class and explicit pointer aliases)
- Private memory (sycl::private_memory)
- Images
- Image Samplers

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

Start with the current Reference guide's “Data access” chapter and compare it with Specification section 4.7 and sycl::private_memory in section 4.9.4.2.3. Review the proposed section hierarchy and identify an improved organization for the updated chapter. Done means reaching agreement on the structure; no files or tests are named.

Written by the indexing model from the issue text.