fix: resolve broken links and create dedicated core concept pages

- Fix broken links:
  - Update Footer GitHub link from generic to project-specific
  - Change Footer "API Reference" label to "Documentation"
  - Fix all /docs/createMachine/ links to /docs/createmachine/ (lowercase)
  - Remove broken robot/debug link reference
  - Ensure all doc links have trailing slashes

- Create dedicated core concept pages:
  - Add concepts-state.md - Understanding State in FSMs
  - Add concepts-transitions.md - Understanding Transitions
  - Add concepts-events.md - Understanding Events
  - Add concepts-guards.md - Understanding Guards
  - Add concepts-actions.md - Understanding Actions

- Update core-concepts.md to serve as overview page with links to detailed concept pages
- Update DocsSidebar to link to new individual concept pages instead of hash sections

Fixes #249

Author: matthewp