Use numpydoc to render the API reference

[vlad-perevezentsev]

This PR switches the documentation build from sphinx.ext.napoleon to numpydoc to render the API reference.
numpydoc renders the Parameters and Returns sections as definition lists out of the box, which lets us remove:

• the custom doc/_static/dpnp-custom.js that reformatted parameter
  descriptions, together with the CSS rules that only supported it;
• the napoleon monkeypatch for namedtuple Returns (numpydoc handles it
  natively).

A small hook keeps the dpnp-specific Limitations section and a some docstrings are adjusted for numpydoc's stricter parser.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/3059/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev8=py314ha0e2e8e_9 ran successfully.
Passed: 1376
Failed: 0
Skipped: 6

[coveralls]

coverage: 78.494%. remained the same — use_numpydocs_in_docs_build into master

[antonwolfy]

Small rendering mismatch vs the legacy build worth a fix: parameter/return types now render upright + semibold instead of italic.

• New: https://intelpython.github.io/dpnp/pull/3059/reference/generated/dpnp.clip.html
• Legacy: https://intelpython.github.io/dpnp/reference/generated/dpnp.clip.html

Cause is purely CSS. numpydoc emits the type as <span class="classifier">, and the italic comes from Sphinxs built-in basic.css (.classifier { font-style: oblique }). NumPys pydata-sphinx-theme @imports basic.css so types are italic, and napoleon previously wrapped the type in <em>. Furo neither imports basic.css nor styles .classifier, and it makes the whole <dt> term semibold, so the type ends up bold/upright.

Proposed fix, add to doc/_static/dpnp-custom.css:

/* numpydoc param/return types: italic like NumPy, not bold */
.classifier {
    font-style: italic;
    font-weight: 400;
}