[SYCL][Doc] Add sycl_ext_intel_maximum_registers spec - #22820

Full disclosure, I copy and pasted the grf_size spec, did a rename from grf_size to maximum_registers, updated the copyright, updated the 'Backend support status' section, and added this line about differences with grf_size.

If there is some other expected difference between the specs please let me know.

This is a good start, but some other things:

• If you intend to merge this PR before it is implemented, change the location of the file to the "proposed" directory. In addition, update the "Status" section with the suggested text from the template for a proposed extension.

• The sentence "A kernel may be defined with at most one of these GRF-setting properties" should be updated in both specs to make it clear that maximum_registers and grf_size are also mutually exclusive. Maybe something like:

  The two properties defined below and the properties from sycl_ext_intel_grf_size are all mutually exclusive. A kernel may be decorated with at most one of these properties.

  (And make "sycl_ext_intel_grf_size" a link.)

• Is there error checking now to see if a kernel is decorated with both grf_size and grf_size_automatic? If so, that error checking should be expanded to all four property when you implement the spec.

Sorry, I rushed through this too much. All good points, I will implement them.

Is there error checking now to see if a kernel is decorated with both grf_size and grf_size_automatic? If so, that error checking should be expanded to all four property when you implement the spec.

Yep there is and I will do this when I implement the extension, thanks.

This section doesn't seem very useful for this extension, so I think it can just be deleted.

done in e10f436, thanks!