Added fixed dimensions indexer for three offsets

[oleksandr-pavlyk]

This PR implements fixed-dimensionality indexer for three offsets. These are useful for writing dpctl-based extensions.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1305/index.html

[coveralls]

coverage: 83.236%. remained the same when pulling bbb2466 on three-strides-fix-dim-indexer into 616c21e on master.

[github-actions]

Array API standard conformance tests for dpctl=0.14.6dev0=py310h7bf5fec_28 ran successfully.
Passed: 478
Failed: 522
Skipped: 119

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.14.6dev0=py310h7bf5fec_40 ran successfully.
Passed: 556
Failed: 444
Skipped: 119