[SYCL][Doc] Add initial draft of the targets registry

[jdnk]

This PR adds a registry containing enumerator values of device targets, features and architectures to be used with the SPV_INTEL_function_variants extension (and other extensions that find it useful). See the README file for details.

The purpose of the PR is to iterate on the registry format before filling in all the entries.

Targets and features come from inspecting compiler output (llc --version and llc --mtriple=<target> -mattr=help). A bulk of the architectures I took directly from sycl_ext_oneapi_device_architecture (mostly the GPUs). Others I added by hand (eg. Intel and ARM CPUs).

Open questions:

• There could be other ways how to categorize the Intel CPU architectures. I opted strictly for the microarchitecture, another possibility is by the processor name / codename (eg. Meteor Lake instead of Redwood Cove + Crestmont). This may apply for other architectures as well.
• Should we add only Intel architectures and let other vendors fill their own? For adoption, it would be better to add as many as we can, but we risk landing on architecture ordering different from what the vendor would prefer.
  ** During version 0 we can pre-add values for other vendors and let vendors take over before "freezing" version 1.