[MINOR][PS][DOCS] Add DataFrame.plot.kde to API reference

### What changes were proposed in this pull request?
1, add `DataFrame.plot.kde` to API reference;
2, sort the plotting function alphabetically;

### Why are the changes needed?
`DataFrame.plot.kde` is a public function in Pandas https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.plot.kde.html

And it was already implemented in PS, but never exposed to users.

### Does this PR introduce _any_ user-facing change?
Yes, doc-only changes

### How was this patch tested?
CI

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #47564 from zhengruifeng/add_kde_to_doc.

Authored-by: Ruifeng Zheng <ruifengz@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/docs/source/reference/pyspark.pandas/frame.rst

@@ -326,14 +326,15 @@ specific plotting methods of the form ``DataFrame.plot.<kind>``.
    :template: autosummary/accessor_method.rst
 
    DataFrame.plot.area
-   DataFrame.plot.barh
    DataFrame.plot.bar
-   DataFrame.plot.hist
+   DataFrame.plot.barh
    DataFrame.plot.box
+   DataFrame.plot.density
+   DataFrame.plot.hist
+   DataFrame.plot.kde
    DataFrame.plot.line
    DataFrame.plot.pie
    DataFrame.plot.scatter
-   DataFrame.plot.density
 
 .. autosummary::
    :toctree: api/

python/docs/source/reference/pyspark.pandas/series.rst

@@ -461,9 +461,9 @@ specific plotting methods of the form ``Series.plot.<kind>``.
    Series.plot.box
    Series.plot.density
    Series.plot.hist
+   Series.plot.kde
    Series.plot.line
    Series.plot.pie
-   Series.plot.kde
 
 .. autosummary::
    :toctree: api/