Synchronization and atomics chapter desing

Currently, the "Atomic Operations" chapter looks like this:

We propose to rename this chapter to "Synchronization and atomics" and implement the following structure:

- Synchronization and atomics
- Barriers and fences ([4.15.1. Barriers and fences](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:barriers-fences))
- ``sycl::device_event `` ([4.15.2. device_event class](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:barriers-fences))
- Atomic references ([4.15.3. Atomic references](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:atomic-references))
- Interaction with host code ([4.15.5. Interaction with host code](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_interaction_with_host_code))

Also, should we include deprecated [Atomic types](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:atom-types-depr) in the reference guide?

We will create the PR with updates soon. Feel free to comment if you have any suggestions on the structure.

Start from the current "Atomic Operations" chapter and compare its organization with the linked SYCL specification sections on barriers, device_event, atomic references, and host interaction. Done means renaming the chapter, applying the proposed subsection structure, and resolving whether deprecated Atomic types belong in the reference guide.

Written by the indexing model from the issue text.