Merge pull request #575 from profunktor/update/mdoc-2.9.0

Update mdoc, sbt-mdoc to 2.9.0

Author: mergify[bot]

project/plugins.sbt

@@ -6,5 +6,5 @@ addSbtPlugin("org.typelevel"  % "sbt-tpolecat"    % "0.5.3")
 addSbtPlugin("com.github.sbt" % "sbt-header"      % "5.11.0")
 addSbtPlugin("org.scalameta"  % "sbt-scalafmt"    % "2.5.5")
 addSbtPlugin("com.47deg"      % "sbt-microsites"  % "1.4.4")
-addSbtPlugin("org.scalameta"  % "sbt-mdoc"        % "2.8.2")
+addSbtPlugin("org.scalameta"  % "sbt-mdoc"        % "2.9.0")
 addSbtPlugin("com.typesafe"   % "sbt-mima-plugin" % "1.1.5")