Refactor dpnp.fix() to reuse dpnp.trunc internally

[vlad-perevezentsev]

This PR rewrites dpnp.fix to reuse dpnp.trunc internally removing the backend implementation of dpnp.fix and aligning the documentation with NumPy

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2722/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev1=py313h509198e_52 ran successfully.
Passed: 1229
Failed: 0
Skipped: 7

[coveralls]

coverage: 81.304% (-0.03%) from 81.337%
when pulling 11ec54a on use_tranc_in_fix_func
into 4d0462d on master.

[antonwolfy]

Thank you @vlad-perevezentsev, LGTM!