fix(scala#19377): show inherited abstract members in dedicated section

Before this commit, inherited abstract members are displayed in
"Inherited members" section, which made it a bit difficult for
developers to figure out which method to be implemented.

I think there are two solution for the issue.

1. display `abstract` modifier for methods in inherited section
2. add a new dedicated section for inherited abstract members

I choose the 2nd solution because

- it is easier for developers to find methods to be implemented in a dedicated section rather than to look
  up abstract ones from all the inherited members
- it requires smaller modification to scaladoc codebase

scaladoc-testcases/src/tests/abstractmembersignatures.scala

@@ -9,6 +9,8 @@ trait TestTrait:
 class TestClass:
   def shouldBeConcrete: Int = 1
 
+abstract class TestInheritedAbstractMembers extends TestTrait
+
 abstract class AbstractTestClass:
   def shouldBeAbstract: Int
   def shouldBeConcrete: Int = 1

scaladoc/src/dotty/tools/scaladoc/renderers/MemberRenderer.scala

@@ -323,6 +323,7 @@ class MemberRenderer(signatureRenderer: SignatureRenderer)(using DocContext) ext
       val (allInherited, allDefined) = nonExperimental.partition(isInherited)
       val (depDefined, defined) = allDefined.partition(isDeprecated)
       val (depInherited, inherited) = allInherited.partition(isDeprecated)
+      val (abstractInherited, concreteInherited) = inherited.partition(isAbstract)
       val normalizedName = name.toLowerCase
       val definedWithGroup = if Set("methods", "fields").contains(normalizedName) then
           val (abstr, concr) = defined.partition(isAbstract)
@@ -335,7 +336,8 @@ class MemberRenderer(signatureRenderer: SignatureRenderer)(using DocContext) ext
 
       definedWithGroup ++ List(
         actualGroup(s"Deprecated ${normalizedName}", depDefined),
-        actualGroup(s"Inherited ${normalizedName}", inherited),
+        actualGroup(s"Inherited ${normalizedName}", concreteInherited),
+        actualGroup(s"Inherited and Abstract ${normalizedName}", abstractInherited),
         actualGroup(s"Deprecated and Inherited ${normalizedName}", depInherited),
         actualGroup(name = s"Experimental ${normalizedName}", experimental)
       )

scaladoc/test/dotty/tools/scaladoc/signatures/AbstractMemberSignaturesTest.scala

@@ -18,10 +18,10 @@ class AbstractMembers extends ScaladocTest("abstractmembersignatures"):
   def runTest = {
     afterRendering {
       val actualSignatures = signaturesFromDocumentation()
-
       actualSignatures.foreach { (k, v) => k match
         case "Abstract methods" => assertTrue(v.forall(_._2 == "shouldBeAbstract"))
         case "Concrete methods" => assertTrue(v.forall(_._2 == "shouldBeConcrete"))
+        case "Inherited and Abstract methods" => assertTrue(v.forall(_._2 == "shouldBeAbstract"))
         case "Classlikes" => assertTrue(v.forall((m, n) => m.contains("abstract") == n.contains("Abstract")))
         case _ =>
       }