Add deprecation warning for the the dpctl.tensor submodule

[ndgrigorian]

As plans have now evolved to move the tensor submodule to dpnp, the tensor submodule will be deprecated in a coming minor version release

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2191/index.html

[coveralls]

coverage: 86.278% (+0.002%) from 86.276%
when pulling 05935ff on deprecate-tensor-submodule
into 70920d3 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.22.0dev0=py310h93fe807_24 ran successfully.
Passed: 1109
Failed: 49
Skipped: 82

[antonwolfy]

Do we need to add tests per both deprecation warnings?

[ndgrigorian]

I don't think there's much of a need for tests, coverage remains the same anyway and it seems to work for me locally

[github-actions]

Array API standard conformance tests for dpctl=0.22.0dev0=py310h93fe807_49 ran successfully.
Passed: 1109
Failed: 49
Skipped: 82