Revise day 3 workshop instructions

.gitlab-ci.yml

docs/workshops/day_3_bdd.md

docs/workshops/images/day_3_-_cucumber_test_suite.png

docs/workshops/images/day_3_-_inspect_element_xpath.png

docs/workshops/images/day_3_-_serenity_report.png

.../workshops/images/day_3_-_spring_petclinic_front_page.png

...rkshops/images/workshop_prep_-_docker_desktop_windows.png

docs/workshops/prep.md

mkdocs.yml