### What changes were proposed in this pull request?
This PR targets to document the Pandas UDF redesign with type hints introduced at SPARK-28264.
Mostly self-describing; however, there are few things to note for reviewers.
1. This PR replace the existing documentation of pandas UDFs to the newer redesign to promote the Python type hints. I added some words that Spark 3.0 still keeps the compatibility though.
2. This PR proposes to name non-pandas UDFs as "Pandas Function API"
3. SCALAR_ITER become two separate sections to reduce confusion:
- `Iterator[pd.Series]` -> `Iterator[pd.Series]`
- `Iterator[Tuple[pd.Series, ...]]` -> `Iterator[pd.Series]`
4. I removed some examples that look overkill to me.
5. I also removed some information in the doc, that seems duplicating or too much.
### Why are the changes needed?
To document new redesign in pandas UDF.
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
Existing tests should cover.
Closes#27466 from HyukjinKwon/SPARK-30722.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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