Move explicit-nulls page to experimental section

Author: odersky

docs/docs/reference/other-new-features/explicit-nulls.md renamed to docs/docs/reference/experimental/explicit-nulls.md

@@ -24,5 +24,5 @@ They can be imported at the top-level if all top-level definitions are @experime
 
 Some experimental language features that are still in research and development can be enabled with special compiler options. These include
 
-* `-Yexplicit-nulls` Enable support for tracking null references in the type system.
+* [`-Yexplicit-nulls`](./explicit-nulls.md). Enable support for tracking null references in the type system.
 

docs/docs/reference/experimental/overview.md

@@ -64,7 +64,6 @@ sidebar:
           - page: docs/reference/other-new-features/targetName.md
           - page: docs/reference/other-new-features/control-syntax.md
           - page: docs/reference/other-new-features/indentation.md
-          - page: docs/reference/other-new-features/explicit-nulls.md
           - page: docs/reference/other-new-features/safe-initialization.md
           - page: docs/reference/other-new-features/type-test.md
           - page: docs/reference/other-new-features/experimental-defs.md
@@ -116,6 +115,7 @@ sidebar:
           - page: docs/reference/experimental/erased-defs.md
           - page: docs/reference/experimental/named-typeargs.md
           - page: docs/reference/experimental/numeric-literals.md
+          - page: docs/reference/experimental/explicit-nulls.md
       - page: docs/reference/syntax.md
       - title: Language Versions
         index: docs/reference/language-versions/language-versions.md