[Doc] Document useSourceContext and improve useSimpleFormIteratorItem documentation

[slax57]

Problem

1. RA v5 introduced the <SourceContext>, which removes the need for several components to clone their children, and also removes the need to call getSource in user-land inside <FormDataConsumer>.
   However, users may still need to use getSource for some more advanced usages, like changing a value programmatically using RHF's setValue.

2. In our doc, we only use useSimpleFormIteratorItem to remove or reorder.
   However, if I need to use the index of my SimpleFormIterator the doc only mentiones getItemLabel which is not enough

Solution

1. Document the new getSource (coming from the <SourceContext>) where it could be needed:

• <ArrayInput>
• <TranslatableInputs>
• Upgrade guide
• <ReferenceManyInput>
• <ReferenceOneInput>

2. Add a section in the SimpleFormIterator doc, after the Props description, titled ‘Getting The Element Index’, and document the total and index values of useSimpleFormIteratorItem

How To Test

• http://localhost:9010/?path=/story/ra-ui-materialui-input-simpleformiterator--use-simple-form-iterator-item
• http://localhost:9010/?path=/story/ra-ui-materialui-input-arrayinput--set-value
• http://localhost:9010/?path=/story/ra-ui-materialui-input-translatableinputs--set-value

Additional Checks

• The PR targets master for a bugfix, or next for a feature
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

Also, please make sure to read the contributing guidelines.