feat(oa): add training-dir workflow and save flow for OnlineAcademy runner

Introduce a simpler training-based workflow for the OnlineAcademy Playwright runner.

Changes included:
- support --training-dir and automatic loading of content.md and training.json
- support --env-file so OA credentials no longer need manual shell sourcing
- resolve asset paths relative to the training directory
- improve human-readable review and execute output with step-by-step progress
- keep the browser open after execute when no save flag is used
- add optional --save flow that clicks 'Opslaan als' and then chooses 'Concept'
- add a concrete user guide for preparing and running training imports
- update handover documentation to reflect the current repo structure and workflow
- align the repo around trainings/<name>/content.md, training.json and assets/
- remove reliance on older pilot/test material in the documented main flow

trainings/hoofdstuk-04-basisqueries/training.json

@@ -0,0 +1,3 @@
+{
+  "url": "https://create.onlineacademy.nl/09953181-2b02-4859-9d2a-137dab417e1b/edit?idsVersion=6"
+}