[SPARK-48893][SQL][PYTHON][DOCS] Add some examples for `linearRegress…

…ion` built-in functions

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

This PR aims to add some extra examples for `linearRegression` built-in functions.

### Why are the changes needed?

- Align the use examples for this series of functions.
- Allow users to better understand the usage of `linearRegression` related methods from sql built-in functions docs(https://spark.apache.org/docs/latest/api/sql/index.html).

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

No.

### How was this patch tested?

Pass GA and Manual testing for new examples.

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

No.

Closes apache#47343 from wayneguow/regr_series.

Authored-by: Wei Guo <guow93@gmail.com>
Signed-off-by: yangjie01 <yangjie01@baidu.com>