From 7 to 8 migration guide link to yaml preconditions

[timtebeek]

Revise the 7 to 8 migration guide now that we have yaml preconditions

Instead of the hint here

rewrite-docs/changelog/8-1-2-Release.md

Lines 17 to 19 in 93da911

	{% hint style="warning" %}
	There is, unfortunately, no way for a YAML recipe to use `Preconditions`. We hope to support such a feature for them eventually, though.
	{% endhint %}

We should probably link here

rewrite-docs/reference/yaml-format-reference.md

Lines 45 to 48 in 93da911

	### Preconditions
	Preconditions are used to limit where a recipe is run.
	This is commonly used to target specific files or directories, but any recipe which is not a `ScanningRecipe` can be used as a precondition.

[timtebeek]

@mike-solomon figured log this as an issue to not forget; didn't immediately pick this up as I thought we might want to go over some of the paragraphs above the tip as well, or whatever you think is best. I don't think the docs necessarily have to reflect all the stages we went through; just have them be applicable to what's now available.