Initiate the tutorial for migrating an sbt project

[adpi2]

The tutorial covers the following steps

• Prerequisite: scala version >2.13, valid compiler plugins, no macro annotations...
• Find the right set of dependencies: most scala 2 dependencies are compatible with scala 3. For the macro libraries it is required to upgrade to a cross-compiled version.
• Optionally fix some incompatibilities with Scalafix (link to the scalafix tutorial Add Scalafix tutorial for migration (sbt) #62)
• Configure the build tool for cross compilation: use sbt 1.4.0-RC2, sbt-dotty plugin and the crossVersions settings
• Find the right set of dotc options depending on the scalac options (link to the compiler options page Add page on compiler options #60)
• Add the dotc migration mode options
• Try to compile with Dotty / fix the errors / compile in 2.13 / run the tests in 2.13 / iterate
• Rewrite with Dotty if necessary
• Leave the Dotty migration mode