docs: ✏️ Update build documentation

Author: gaeljw

CONTRIBUTING.md

@@ -12,6 +12,5 @@ Please use this Github project for contributing, either through an issue or a Pu
 
 These pages aim to help Cucumber Scala developers understand the codebase.
 
-- [Building](docs/building.md)
-- [Project structure](docs/project_structure.md)
+- [Build](docs/build.md)
 - [Scala implementation details](docs/scala_implementation.md)

docs/build.md

@@ -0,0 +1,82 @@
+# Build
+
+To compile and test the whole project you can run the following command:
+
+```shell
+$ sbt clean compile test
+```
+
+## Project structure
+
+The project contains two functional subprojects:
+- `cucumber-scala`: contains the codebase of the Cucumber Scala implementation
+- `examples`: contains a sample project
+
+Each of these subproject is also derived for each target Scala version. See below.
+
+## Cross compilation
+
+Cross compilation to multiple Scala versions is handled by the [sbt-projectmatrix](https://github.com/sbt/sbt-projectmatrix) plugin.
+
+The target versions are defined in the `build.sbt` on each sub-project:
+```scala
+project
+  ...
+  .jvmPlatform(scalaVersions = Seq(scala3, scala213, scala212))
+```
+
+A sbt sub-project is generated for each targeted Scala version with the version as a suffix
+(with Scala 2.13 being the current default version):
+
+```shell
+$ sbt projects
+...
+[info]     cucumberScala
+[info]     cucumberScala2_12
+[info]     cucumberScala3
+[info]     examples
+[info]     examples2_12
+[info]     examples3
+[info]   * root
+```
+
+### Sources
+
+Sources should most of the time be compatible for all target Scala versions in order to ease maintenance.
+
+However, if needed, it's possible to define different sources for each Scala version.
+
+These version-specific sources should be put in a directory called `src/main/scala-<version>` and the `build.sbt`
+should declare them as additional sources directories like:
+```scala
+Compile / unmanagedSourceDirectories ++= {
+  val sourceDir = (Compile / sourceDirectory).value
+  CrossVersion.partialVersion(scalaVersion.value) match {
+    case Some((2, n)) =>
+      Seq(sourceDir / "scala-2")
+    case Some((3, 0)) =>
+      Seq(sourceDir / "scala-3")
+    case _ =>
+      Seq()
+  }
+}
+```
+
+The same can be done for tests.
+
+## Language traits generation
+
+The language traits (`io.cucumber.scala.EN` for instance) are generated automatically at compile time.
+
+The `project` meta project defines a `I18nGenerator` object responsible for generating their content.
+
+At compilation, this generated content is injected as source files thanks to a sbt source generator:
+```scala
+// Generate I18n traits
+Compile / sourceGenerators += Def.task {
+  val file =
+    (Compile / sourceManaged).value / "io/cucumber/scala" / "I18n.scala"
+  IO.write(file, I18nGenerator.i18n)
+  Seq(file)
+}.taskValue
+```