docs: fix broken intersphinx links

[Ekin-Kahraman]

Summary

• Add napoleon_type_aliases for short-form types (ndarray, pd.DataFrame, pd.Series, np.ndarray, pandas.Index) so Napoleon maps them to fully qualified intersphinx targets
• Remove pd.Series, pd.DataFrame, pandas.core.frame.DataFrame, and ndarray from nitpick_ignore since intersphinx now resolves them correctly
• Add anndata._core.anndata.AnnData to nitpick_ignore (autodoc resolves the runtime module path for base-class references, which intersphinx cannot remap)

Closes #437

Test plan

• make html in docs/ builds with zero warnings

🤖 Generated with Claude Code

[grst]

<unknown>:1: WARNING: py:class reference target not found: pandas.core.frame.DataFrame [ref.class]
<unknown>:1: WARNING: py:class reference target not found: pandas.core.frame.DataFrame [ref.class]
<unknown>:1: WARNING: py:class reference target not found: pandas.core.frame.DataFrame [ref.class]
<unknown>:1: WARNING: py:class reference target not found: pandas.core.frame.DataFrame [ref.class]

[Ekin-Kahraman]

Thanks for the review @grst!

pandas.core.frame.DataFrame warnings -- Fixed in 77987f4. The issue is that autodoc resolves pd.DataFrame type annotations to the runtime internal path pandas.core.frame.DataFrame, but the pandas intersphinx inventory only exposes the public API path pandas.DataFrame. This is the same pattern as the existing anndata._core.anndata.AnnData entry, so I added it to nitpick_ignore with an explanatory comment.

CI failure (Python 3.11) -- The "Compile docs" step is the only thing failing, and it's the same step that's been failing on main too (every scheduled run since at least April 2 has the same failure). On main the warning is the pre-existing anndata._core.anndata.AnnData one that this PR already addresses. With the pandas fix pushed, this PR should now resolve both warnings and get CI green.