[SPARK-53182][PYTHON][DOCS] Fix broken and missing links in PySpark DataFrames user guide

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

This PR fixes two small issues in the PySpark DataFrames user guide:

1. **Replaces a broken external link** to a section on data manipulation. The previous link pointed to an outdated Databricks-hosted page. It is now replaced with a working internal link to Chapter 3 of the PySpark user guide:
   [Chapter 3: Function Junction - Data manipulation with PySpark](https://spark.apache.org/docs/latest/api/python/user_guide/dataprep.html)

2. **Adds a missing link** for the section on saving DataFrames to persistent storage. The text previously said “TODO: add link.” This has been replaced with a correct reference to:
   [Chapter 7: Load and Behold - Data loading, storage, file formats](https://spark.apache.org/docs/latest/api/python/user_guide/loadandbehold.html)

### Why are the changes needed?

These changes improve the quality and usability of the documentation by fixing a broken link and completing a placeholder that may confuse users. It ensures readers are directed to up-to-date, relevant internal documentation instead of an outdated or unavailable external resource.

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

Yes. It updates two markdown cells in the `dataframes.ipynb` user guide notebook, affecting how users navigate to related documentation when reading the generated HTML docs.

### How was this patch tested?

The documentation was built locally using `make html` in the `python/docs` directory. The rendered output for the notebook was reviewed in a browser to confirm the links appear and function correctly.

<img width="1451" height="225" alt="Screenshot 2025-08-07 at 11 53 19 AM" src="https://github.com/user-attachments/assets/41d3342d-0637-48e5-b010-e2b6e5bc5630" />
<img width="1435" height="231" alt="Screenshot 2025-08-07 at 11 52 54 AM" src="https://github.com/user-attachments/assets/466be073-1f90-4cf1-b1e7-ad03f5bbe21a" />

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

No.

Closes #51851 from jonnycomes/fix-doc-user-guide-dataframes-links.

Authored-by: Jonny Comes <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: jonnycomes

python/docs/source/user_guide/dataframes.ipynb

@@ -528,7 +528,9 @@
                 "\n",
                 "Let's look at some ways we can transform our DataFrames.\n",
                 "\n",
-                "For more detailed information, please see the section about data manipulation, [Chapter 6: Function Junction - Data manipulation with PySpark](https://databricks-eng.github.io/pyspark-cookbook/07-dataprep.html).\n",
+                "For more detailed information, please see the section about data manipulation, ",
+                "[Chapter 3: Function Junction - Data manipulation with PySpark](https://spark.apache.org/docs/latest/api/python/user_guide/dataprep.html).\n",
+                "\n",
                 "\n",
                 "### Rename columns\n",
                 "\n",
@@ -686,8 +688,8 @@
                 "## Save DataFrame to Persistent Storage\n",
                 "\n",
                 "There are several ways to save a DataFrame to persistent storage in PySpark.\n",
-                "For more detailed information about saving data to your local environment,\n",
-                "please see the section about Data Loading (TODO: add link)."
+                "For more detailed information about saving data locally, see ",
+                "[Chapter 7: Load and Behold - Data loading, storage, file formats](https://spark.apache.org/docs/latest/api/python/user_guide/loadandbehold.html).\n"
             ]
         },
         {
@@ -795,3 +797,4 @@
     "nbformat": 4,
     "nbformat_minor": 5
 }
+