docs: trim README and add the BPMN process diagram as SVG

[emaarco]

Shortens the README and makes the process model its lead visual, so the repo reads well as a template for future backend-only variants (which won't have a frontend screenshot but will share the model).

• Add the bike-leasing BPMN process as the hero image, exported to SVG (replaces the old bike-leasing.png; docs/README.md reference updated).
• Cut ~40% of the prose: drop the "Technology choices" ADR list that duplicated the table in docs/README.md, condense the scenario, and merge redundant sections.
• Plainer, more engaging wording throughout (e.g. "architecture kept honest by tests"), and no em-dashes in the body.