Update README.md

…o remember at-a-glance:

- Remove card #2 on 'Complex theme' and rename 'Puzzle theme'
- Remove following fields from the renamed 'Complex theme':
  - Remove '✔ What does this syntax do?' field, as well as removing:
    - ----✄-- Hide on front: Syntax (inline code)
    - »»♻»» Show as reference: Syntax (inline code)
    - ----✄-- Hide on front: Sample (code block or image)
    - »»♻»» Show as reference: Sample (code block or image)
- Rename fields including »»♻»» to »»♻«« and reset the card templates
- Easier to understand documentation for the themes (I hope)
- Change a few README urls