+doc add http (experimental) migration guide to ease upgrading DSLs (#20860)

* +doc add http (experimental) migration guide to ease upgrading DSLs

* Update migration-guide-2.4.x-experimental.rst

akka-docs/rst/scala/http/index.rst

@@ -15,3 +15,4 @@ Akka HTTP
    server-side-https-support
    migration-from-spray
    migration-from-old-http-javadsl
+   migration-guide-2.4.x-experimental

akka-docs/rst/scala/http/migration-guide-2.4.x-experimental.rst

@@ -0,0 +1,26 @@
+Migration Guide between experimental builds of Akka HTTP (2.4.x)
+================================================================
+
+General notes
+-------------
+Please note that Akka HTTP consists of a number of modules, most notably `akka-http-core`
+which is **stable** and won't be breaking compatibility without a proper deprecation cycle,
+and `akka-http` which contains the routing DSLs which is **experimental** still.
+
+The following migration guide explains migration steps to be made between breaking
+versions of the **experimental** part of Akka HTTP. 
+
+.. note:: 
+  Please note that experimental modules are allowed (and are expected to) break compatibility
+  in search of the best API we can offer, before the API is frozen in a stable release. 
+  
+  Please read :ref:`BinCompatRules` to understand in depth what bin-compat rules are, and where they are applied.
+
+Akka HTTP 2.4.7 -> 2.4.8
+------------------------
+
+`SecurityDirectives#challengeFor` has moved
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The `challengeFor` directive was actually more like a factory for `HttpChallenge`,
+thus it was moved to become such. It is now available as `akka.http.javadsl.model.headers.HttpChallenge#create[Basic|OAuth2]`
+for JavaDSL and `akka.http.scaladsl.model.headers.HttpChallenges#[basic|oAuth2]` for ScalaDSL.