[Docs] return back preparing Data

[Nir-An]

Add missing information about preparing Data

related to discussion #272

originally from PR 247

[rubenvanassche]

Still in docs:

laravel-data/docs/advanced-usage/pipeline.md

Line 67 in 8b4d79b

Sometimes you need to make some changes to the payload after it has been normalized, but before they are sent into the data pipeline. You can do this using the `prepareForPipeline` method as follows:

But seemingly our docs aren't generating

[Nir-An]

OK, thanks!

we got some questions about this issue and we don't have a reference to send them to.