update pwm constraint values for easy bot performance

[5iri]

The previous defaults were too fast for the current bot design. Based on testing on Wall‑Ev2.7, this PR updates motor PWM bounds and documents the new recommended values.

Changes

• Self‑Balancing: update duty cycle bounds
  • 6_self_balancing/main/self_balancing.c: #define MAX_PWM (60.0f)
  • 6_self_balancing/main/self_balancing.c: #define MIN_PWM (30.0f)
• Line‑Following: update base and bounds
  • 5_line_following/main/line_following.c: int optimum_duty_cycle = 40;
  • 5_line_following/main/line_following.c: int lower_duty_cycle = 10;
  • 5_line_following/main/line_following.c: int higher_duty_cycle = 55;
• Documentation:
  • Add Wall‑Ev2.7 PWM settings in root README.md
  • Add defaults sections in 6_self_balancing/README.md and 5_line_following/README.md

[Shankari02]

I suppose this is no more required as the problem was found out to be something else?