Update datafusion-examples/README.md occording to the new examples format

[cj-zhukov]

part of ##18142

I think README.md needs to be updated to use the examples in this new format. Use table format instead, so it's easier to see which command to run for which example.

Example

Group	Example	Subcommand	File Path	Description
udf	udf	adv_udaf	path-to-file.rs	Define and invoke a more complicated User Defined Aggregate Function (UDAF)
udf	udf	udaf	path-to-file.rs	Define and invoke a User Defined Aggregate Function (UDAF)

How to run

cargo run --example udf -- [adv_udaf|udaf]

Originally posted by @Jefffrey in #18289 (comment)

[cj-zhukov]

take