Add 'Lezione 06'

src/06_git-workflow/00_index.md

src/06_git-workflow/01_gitflow.md

src/06_git-workflow/02_hosting-centralizzato.md

src/06_git-workflow/03_gerrit.md

src/06_git-workflow/04_strumenti-opensource/00_index.md

...t-workflow/04_strumenti-opensource/01_build-automation.md

...6_git-workflow/04_strumenti-opensource/02_bug-tracking.md

src/SUMMARY.md