Add vulpix modification docs

Author: anatoliykmetyuk

docs/docs/contributing/debugging.md renamed to docs/docs/contributing/codebase/debugging.md

@@ -0,0 +1,30 @@
+# Test Vulpix Framework
+If you are modifying the Vulpix framework and need a playground with dummy tests to try out your modifications, do the following.
+
+Create the directory structure for the playground:
+
+```bash
+mkdir -pv tests/playground/run tests/playground/neg
+echo "stuff" > tests/playground/neg/Sample.scala
+echo 'object Test { def main(args: Array[String]): Unit = {println("Hi")} }' > tests/playground/run/Sample.scala
+```
+
+In `CompilationTests.scala`:
+
+```scala
+  @Test def exampleNeg: Unit = {
+    implicit val testGroup: TestGroup = TestGroup("exampleNeg")
+    compileFilesInDir("tests/playground/neg", defaultOptions).checkExpectedErrors()
+  }
+
+  @Test def exampleRun: Unit = {
+    implicit val testGroup: TestGroup = TestGroup("exampleRun")
+    compileFilesInDir("tests/playground/run", defaultOptions).checkRuns()
+  }
+```
+
+SBT:
+
+```scala
+testOnly dotty.tools.dotc.CompilationTests -- *example*
+```