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
Migrated from D1220 (view on Phabricator)