Add common mistakes section to errors-and-debugging.md

[travisjneuman]

Description

The errors and debugging concept doc could be more helpful with a "Top 5 Beginner Mistakes" section showing the most common errors new learners encounter (IndentationError, NameError from typos, missing colons, etc.) with examples and fixes.

Acceptance Criteria

• Add a section to concepts/errors-and-debugging.md with 5 common mistakes
• Each mistake includes: the error message, the broken code, the fix, and why it happens
• Keep language beginner-friendly

[travisjneuman]

Resolved in 36dceb1: Added Top 5 Beginner Mistakes section to concepts/errors-and-debugging.md with examples and fixes.