doc: add api-docs and website documentation

chimney/src/main/scala-3/io/scalaland/chimney/dsl/TransformerInto.scala

@@ -211,6 +211,13 @@ final class TransformerInto[From, To, Overrides <: TransformerOverrides, Flags <
   )(using IsFunction.Of[Ctor, To]): TransformerInto[From, To, ? <: TransformerOverrides, Flags] =
     ${ TransformerIntoMacros.withConstructorImpl('this, 'f) }
 
+  /** Require that all fields of the source object except fields mentioned in `selectorFrom` are used in the
+    * transformation. and fail compilation otherwise.
+    *
+    * @param selectorFrom
+    *   exception fields that are not required to be used in the transformation
+    * @return
+    */
   transparent inline def requireSourceFieldsUsedExcept(
       inline selectorFrom: From => Any*
   ): TransformerInto[From, To, ? <: TransformerOverrides, Flags] =

docs/docs/supported-transformations.md

@@ -759,6 +759,41 @@ If the flag was enabled in the implicit config it can be disabled with `.disable
     // Consult https://chimney.readthedocs.io for usage examples.
     ```
 
+### Require source fields to be used
+
+If you want to enforce that every field of the source type is used in the transformation, you can enable the
+`.requireSourceFieldsUsedExcept` setting. This setting also allows you to add fields you'd like to ignore:
+
+!!! example
+
+    ```scala
+    //> using dep io.scalaland::chimney::{{ chimney_version() }}
+    import io.scalaland.chimney.dsl._
+
+    case class Source(a: String, b: Int, c: String)
+    case class Target(a: String)
+
+    Source("value", 512, "anotherValue")
+      .into[Target]
+      .requireSourceFieldsUsedExcept()
+      .transform
+    // Chimney can't derive transformation from Source to Target
+    // 
+    // Target
+    //   field(s) b, c of Source are required to be used in the transformation but are not used!
+    //
+    // Consult https://chimney.readthedocs.io for usage examples.
+
+    pprint.pprintln(
+        Source("value", 512, "anotherValue")
+          .into[Target]
+          .requireSourceFieldsUsedExcept(_.b, _.c)
+          .transform
+    )
+    // expected output:
+    // Target(a = "value")
+    ```
+
 ### Writing to Bean setters
 
 If we want to write to `def setFieldName(fieldName: A): Unit` as if it was `fieldName: A` argument of a constructor -