[FEEL] Add a tip on how to get the current index inside a loop

[saig0]

Description

In some cases, I have a FEEL expression with a for-loop and need to get the current element's index.

There is a trick: count(partial) calculates the current index.

Let's share this tip in the FEEL documentation.

Relevant docs:

• https://docs.camunda.io/docs/components/modeler/feel/language-guide/feel-control-flow/#for-loops

Related Slack discussion: https://camunda.slack.com/archives/C01QYD808A3/p1742481110327559