Refactor the getting started guides

• remove the manual setup guide since we do not want to promote (and maintain) this approach any more,
• few rewordings in getting-started.rst iteself
• in the developer-setup guide:
  • rewordings
  • copy the 'install depencencies' from the manual-setup guide,
  • add the missing step bin/update
  • remove the docker setup part (been integrated in the swh-docker-dev's README)
  • add a section on unit tests