Add a docs page for betterFors experimental feature

docs/_docs/reference/experimental/better-fors.md

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+---
+layout: doc-page
+title: "Better fors"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/experimental/better-fors.html
+---
+
+The `betterFors` language extension improves the usability of `for`-comprehensions.
+
+The extension is enabled by the language import `import scala.language.experimental.betterFors` or by setting the command line option `-language:experimental.betterFors`.
+
+The biggest user facing change is the new ability to start `for`-comprehensions with with aliases. This means that the following previously invalid code is now valid:
+
+```scala
+for
+  as = List(1, 2, 3)
+  bs = List(4, 5, 6)
+  a <- as
+  b <- bs
+yield a + b
+```
+
+The desugaring of this code is the same as if the aliases were introduced with `val`:
+
+```scala
+val as = List(1, 2, 3)
+val bs = List(4, 5, 6)
+for
+  a <- as
+  b <- bs
+yield a + b
+```
+
+Additionally this extension changes the way `for`-comprehensions are desugared. The desugaring is now done in a more intuitive way and the desugared code can be more efficient, because it avoids some unnecessary method calls. There are two main changes in the desugaring:
+
+1. **Simpler Desugaring for Pure Aliases**:
+    When an alias is not followed by a guard, the desugaring is simplified. The last generator and the aliases don't have to be wrapped in a tuple, and instead the aliases are simply introduced as local variables in a block with the next generator.
+    **Current Desugaring**:
+      ```scala
+      for {
+        a <- doSth(arg)
+        b = a
+      } yield a + b
+      ```
+      Desugars to:
+      ```scala
+      doSth(arg).map { a =>
+        val b = a
+        (a, b)
+      }.map { case (a, b) =>
+        a + b
+      }
+      ```
+    **New Desugaring**:
+      ```scala
+      doSth(arg).map { a =>
+        val b = a
+        a + b
+      }
+      ```
+    This change makes the desugaring more intuitive and avoids unnecessary `map` calls, when an alias is not followed by a guard.
+
+2. **Avoiding Redundant `map` Calls**:
+    When the result of the `for`-comprehension is the same expression as the last generator pattern, the desugaring avoids an unnecessary `map` call. but th eequality of the last pattern and the result has to be able to be checked syntactically, so it is either a variable or a tuple of variables.
+    **Current Desugaring**:
+      ```scala
+      for {
+        a <- List(1, 2, 3)
+      } yield a
+      ```
+      Desugars to:
+      ```scala
+      List(1, 2, 3).map(a => a)
+      ```
+    **New Desugaring**:
+      ```scala
+      List(1, 2, 3)
+      ```
+
+For more details on the desugaring scheme see the comment in [`Desugar.scala#makeFor`](https://github.com/scala/scala3/blob/main/compiler/src/dotty/tools/dotc/ast/Desugar.scala#L1928).