[REM] workflow: Remove workflow engine, documentation and tests

addons/point_of_sale/models/pos_order.py

addons/purchase/models/mail_mail.py

addons/purchase/models/purchase.py

addons/web/controllers/main.py

addons/web/static/src/js/framework/data.js

addons/web/static/src/js/framework/model.js

addons/web/static/src/js/framework/view.js

addons/website_crm_partner_assign/models/crm_lead.py

doc/api_integration.rst

doc/howtos/backend.rst

doc/howtos/backend/exercise-state-workflow

doc/howtos/backend/exercise-state-workflow-actions

doc/howtos/backend/exercise-state-workflow-automatic

doc/howtos/backend/series

doc/reference.rst

doc/reference/actions.rst

doc/reference/data.rst

doc/reference/guidelines.rst

doc/reference/mixins.rst

doc/reference/security.rst