Update Scala CLI as scala related docs

[Gedochao]

Closes #3147
Closes #3132

What this does:

• the installation doc now includes a section about installing Scala CLI as the scala command via the Scala 3 command
• the README mentions Scala CLI's status as the official language runner
• filled in some missing scenarios in the old runner to Scala CLI as scala migration guide
• the old runner to Scala CLI as scala migration guide now uses the correct commands for both runners in a Scala 3.5.0+ installation (scala & scala_legacy)
  • sclicheck has been modified to allow for those to be tested in the docs-tests workflow on the CI
  • the scala command in docs-tests is currently just an alias for scala-cli, rather than a 1:1 use of the scala wrapper from scala:3.5.0 installation
  • scalac and scala_legacy are currently launched through their main classes with scala-cli, rather than via an actual installation (although that's largely the same, just slower)
  • a potential follow-up could swap scala, scalac and scala_legacy to an actual coursier installation of the default Scala version, but that's out of scope for this PR
  • created a ticket: Add support for calling commands from an actual Scala language installation in sclicheck #3156
• misc related docs improvements

[Gedochao]

cc @hamzaremmal