"Built-in functions for SYCL host and device" chapter desing

Currently, the [SYCL Built-in functions for SYCL host and device](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sycl:builtins) chapter is not represented in the Reference Guide.

We propose to add it with the following structure:

- Built-in functions
- Function objects ([4.17.1. Function objects](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:function-objects))
- Group functions ([4.17.2. Group functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:group-functions))
- Group algorithms library ([4.17.3. Group algorithms library](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:algorithms))
- Math functions ([4.17.4. Math functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_math_functions))
- Native precision math functions ([4.17.5. Native precision math functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_native_precision_math_functions))
- Half precision math functions ([4.17.6. Half precision math functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_half_precision_math_functions))
- Integer functions ([4.17.7. Integer functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:integer-functions))
- Common functions ([4.17.8. Common functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_common_functions))
- Geometric functions ([4.17.9. Geometric functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:geometric-functions))
- Relational functions ([4.17.10. Relational functions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_relational_functions))

This chapter will appear last in the contents bar.

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

Review the current SYCL Reference Guide contents and the linked SYCL 2020 specification chapter, especially sections 4.17.1–4.17.10. The proposed chapter structure and placement need maintainer discussion first; done means an agreed organization that can guide a follow-up documentation PR.

Written by the indexing model from the issue text.