force usage of custom scalameta for mdoc

Coursier seems to prefer mdoc's scalameta 4.8.15 over core2_13's scalameta
4.8.15+150-a7c0baa0-SNAPSHOT. Adding scalameta as a top-level docs2_13
libraryDependencies does change this behavior, so I suspect Coursier is
following nearest-wins semantics for some reason, while we expect it to
following latest-wins semantics.

http://eed3si9n.com/dependency-resolver-semantics#coursiers-latest-wins-semantics

By using dependencyOverrides, we ensure that docs run are generated with the
actual scalameta version used in scalafix, rather than the one used in mdoc.

build.sbt

@@ -383,7 +383,8 @@ lazy val docs = projectMatrix
     scalacOptions += "-Wconf:msg='match may not be exhaustive':s", // silence exhaustive pattern matching warning for documentation
     scalacOptions += "-Xfatal-warnings",
     mdoc := (Compile / run).evaluated,
-    libraryDependencies += metaconfigDoc
+    libraryDependencies += metaconfigDoc,
+    dependencyOverrides += scalameta // force eviction of mdoc transitive dependency
   )
   .defaultAxes(VirtualAxis.jvm)
   .jvmPlatform(scalaVersions = Seq(scala213))