"Host task" chapter desing

Currently, the [Host tasks](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#subsec:interfaces.hosttasks) chapter is not represented in the Reference Guide.

We propose to add it with the following structure:

- Host task
- Host task basic concept ([4.10.1. Overview](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:interfaces.hosttasks.overview))
- ``sycl::interop_handle`` ([4.10.2. Class interop_handle](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#subsec:interfaces.hosttasks.interophandle))
- Additions to the ``sycl::handler`` class ([4.10.3. Additions to the handler class](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#subsec:interfaces.hosttask.handler))

This chapter will be added between the "Expressing Parallelism" and "Error Handling" chapters in the current structure:

Could you please review and provide your opinion and recommendations for this chapter?
We will create PR after a discussion.

Review the proposed Host task structure against the linked SYCL 2020 sections 4.10.1–4.10.3 and the current Reference Guide chapter order. Done means recommendations are recorded and the chapter scope and placement are agreed before a PR is created.

Written by the indexing model from the issue text.