Integrate the specification into the oneDPL documentation

[akukanov]

On top of #2621, this PR makes changes to build the oneDPL specification as part of the overall project documentation.

• Add it as a top-level item to the table of content
• Remove or replace missing variables
• Change the version to 1.5-provisional, decoupled from the oneAPI specification version
• Add a missing copyright notice.

[akukanov]

These two copyright notices are consistent within the oneDPL specification, with two exceptions:

• sycl_kernels_api.rst misses the "contributors" notice;
• parallel_range_api.rst misses the Intel Corporation notice

The marked change addresses the first one, but the second one is I believe intentional. The logic behind this is that after 2022 Intel acted as one but not the only contributor to the specification (technically, though, all modifications of the oneDPL spec came from Intel, according to the commit log). Since the parallel range algorithms were added to the spec only in 2024, there is no Intel copyright notice.

Perhaps it makes sense to change the second notice to "UXL Foundation Contributors". As for the years in the first notice, I think we can either leave these as-is, change to 2019-2023 (assuming that it's transmitted to UXL then), or change to the year of the last modification. I have some preference for 2023.

[timmiesmith]

LGTM.

[akukanov]

I think this document is best positioned as a provisional specification at this point, There is still at least one change in progress, and generally, I think we need some formal decision to "release" the new specification.

[danhoeflinger]

LGTM