[SPARK-45966][DOCS][PS] Add missing methods for API reference

### What changes were proposed in this pull request?

This PR proposes to add missing methods for API reference.

### Why are the changes needed?

For better API reference, we should reflect the actual status into the documentation.

### Does this PR introduce _any_ user-facing change?

No API changes, but user-facing documentation will be improved.

### How was this patch tested?

The existing CI, especially the documentation build should pass.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #43860 from itholic/SPARK-45966.

Authored-by: Haejoon Lee <haejoon.lee@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: itholic

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

@@ -105,7 +105,9 @@ Missing Values
    Index.fillna
    Index.dropna
    Index.isna
+   Index.isnull
    Index.notna
+   Index.notnull
 
 Conversion
 ~~~~~~~~~~
@@ -190,6 +192,10 @@ Categorical components
    CategoricalIndex.as_ordered
    CategoricalIndex.as_unordered
    CategoricalIndex.map
+   CategoricalIndex.equals
+   CategoricalIndex.max
+   CategoricalIndex.min
+   CategoricalIndex.tolist
 
 .. _api.multiindex:
 

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

@@ -214,6 +214,7 @@ Missing data handling
 
    Series.backfill
    Series.bfill
+   Series.ffill
    Series.isna
    Series.isnull
    Series.notna