[SYCL][DOC] Update dot_acc extension spec

[gmlueck]

Update the sycl_ext_oneapi_dot_accumulate extension spec to:

• Use the latest specification template.
• Document the "packed" APIs. These were previously shown in the "sample header" section, but there was no description.
• Deprecate the old "packed" overloads and introduce new overloads with more descriptive names like doc_acc_4x8packed_su. This new names are similar to the OpenCL C naming style for the equivalent functions.
• Deprecate the API that takes two unsigned vectors and a signed accumulator. Replace this with an API that takes two unsigned vectors and an unsigned accumulator. This matches the SPIR-V extension and also the OpenCL API.

[gmlueck]

@intel/dpcpp-specification-reviewers this is ready for re-review now.

[github-actions[bot]]

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