Add classic operator in TaskFlow API tutorial

[casassg]

Description

TaskFlow API tutorial should add an example to use a classic operator (example: EmailOperator) so that users know that it can be leveraged.

Alternatively, it should add references to how to add dependencies (implicit or explicit) to classic operators.

Use case / motivation

It's not super clear how can TaskFlow API be used with existing operators (aka PostgresOperator, EmailOperator...). Adding an example, will facilitate users to get a picture of what can be done with this.

[anilkulkarni87]

@casassg
Here is an example I have created and this dag implements taskgroup, taskflow with classic operators. Please take a look:
Postgres DAG with Taskgroup and Taskflow

Here is how the dependency looks like on UI:

[casassg]

This is good. I think the example should include using .outputs though (aka implicit dependencies for traditional operators).

[josh-fell]

@vikramkoka @casassg Is this type of example still desired? I can help contribute if it is.

[josh-fell]

@eladkal @vikramkoka Following up on this. Feel free to assign to me.

[josh-fell]

@eladkal Perhaps this has already been taken care of looking at the latest TaskFlow API Tutorial doc (starting here). WDYT?

[josh-fell]

@eladkal #19214 was merged earlier. I believe this issue can be closed if you agree. This PR (and even the existing tutorial doc) has a few examples of classic operator use with TaskFlow API and generally with AIP-31.

[josh-fell]

Does anyone else agree that this can be closed now via existing docs + merging of #19124?