Merge language-reference-stable into main (#16243)

This solves all merge conflicts between `language-reference-stable` and
`main`
Solves the first part of #16238

Author: bishabosha

docs/_docs/reference/dropped-features/this-qualifier.md

@@ -29,5 +29,3 @@ This can cause problems if a program tries to access the missing private field v
       // [C] needed if `field` is to be accessed through reflection
     val retained = field * field
 ```
-
-

docs/_docs/reference/other-new-features/indentation.md

@@ -506,4 +506,4 @@ ColonArgument    ::=  colon [LambdaStart]
                       indent (CaseClauses | Block) outdent
 LambdaStart      ::=  FunParams (‘=>’ | ‘?=>’)
                    |  HkTypeParamClause ‘=>’
-```
+```