Remove coverage docs from usage. Describe code coverage process in internals

Author: pikinier20

docs/_docs/internals/coverage.md

@@ -0,0 +1,37 @@
+---
+layout: doc-page
+title: "Code Coverage for Scala 3"
+---
+
+## Instrument code for coverage analysis
+
+[PR#13880](https://github.com/lampepfl/dotty/pull/13880) has implemented code coverage support for Dotty.
+In general, code coverage "instruments" the program at compile time: code is inserted to record which statement are called. This does not change the behavior of the program. Also, a list of all the coverable statements is produced.
+
+To use this feature, add the compile option `-coverage-out:DIR`, where `DIR` is the destination of the measurement files.
+
+You can also set `-sourceroot:PATHS_ROOT` to customize how the path of your source files are resolved.
+Note that `-sourceroot` also sets the root path of the SemanticDB files.
+
+## Details: how the code is instrumented
+
+When the `-coverage-out` option is set, a new phase `instrumentCoverage` runs, just before `firstTransform`.
+For a carefully selected list of tree types, it adds a call to `scala.runtime.Invoker.invoked(statementId, DIR)`.
+
+For instance, this code:
+```
+def method() =
+  println(f())
+```
+
+with `-coverage-out:target/cov` be turned to
+```
+def method() =
+  Invoker.invoked(2, "target/cov")
+  println({
+    Invoker.invoked(1, "target/cov")
+    f()
+  })
+```
+
+At the end of the phase, the list of all the instrumented statements is serialized to the file `DIR/scoverage.coverage`.