[SPARK-35647][PYTHON][DOCS] Restructure User Guide in PySpark documentation

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to restructure User Guide in PySpark documentation for pandas APIs on Spark.

Before

After

Note that I mostly just moved the contents around except minor changes:

• Removing some questions in FAQ that don't make sense in Apache Spark
• Rename a subtitle "Working with pandas and PySpark" to "From/to pandas and PySpark DataFrames"

For renaming Koalas to either pandas-on-Spark or pandas APIs on Spark, it will be done at SPARK-35591

Why are the changes needed?

For better readability.

Does this PR introduce any user-facing change?

Yes, it restructures the documentation as shown above.

How was this patch tested?

I manually built the docs and tested.

[HyukjinKwon]

cc @itholic @ueshin @xinrong-databricks FYI

[SparkQA]

Test build #139479 has finished for PR 32820 at commit 3fd1721.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/44002/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/44002/

[xinrong-meng]

From/to pandas and PySpark? From/to pandas/PySpark? Conversion from/to pandas and PySpark? DataFrames conversion from/to pandas and PySpark?
I just wanted to provide more options, the existing one looks good as well. :)

[HyukjinKwon]

lol. I think it's fine either way. I intentionally wrote PySpark and pandas DataFrames becuase we're already in the same PySpark.

[xinrong-meng]

LGTM! Thanks.

[HyukjinKwon]

Thanks @xinrong-databricks.

Merged to master.