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