Add docs for flattening_enabled and flattening_max_depth

[aaronsteers]

I searched the docs and couldn't find any explanation of the flattening feature. We should add something, either under the stream maps docs page, or on another page.

sdk/singer_sdk/helpers/capabilities.py

Lines 29 to 43 in 3998ac1

	FLATTENING_CONFIG = PropertiesList(
	Property(
	"flattening_enabled",
	BooleanType(),
	description=(
	"'True' to enable schema flattening and automatically expand nested "
	"properties."
	),
	),
	Property(
	"flattening_max_depth",
	IntegerType(),
	description="The max depth to flatten schemas.",
	),
	).to_dict()

Was discovered here:

• Add record flattening feature from SDK MeltanoLabs/meltano-map-transform#24

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[edgarrmondragon]

Still relevant