Add Curvilinear Perspective Docs

Here's documentation and an associated picture for the new Curvilinear Perspective Assistant introduced in graphics/krita!1960 (merged)

Note: I'm not sure what exact release the feature will be included in, as it is a new feature, so I defaulted it to 5.3 in the documentation. Aswell, I'm not sure when this request itself should be merged, since it will likely be a while until the feature is included in a release. As such, feel free to ask for any changes and/or sit on this until the documentation change is needed.