Add narrative text and code cleanup to Euclid clusters tutorial

[jkrick]

• Conformed to template
• Added introductory paragraph explaining why Euclid is well-suited for galaxy cluster science
• Added narrative text throughout all sections explaining the scientific motivation behind each step
• Hid function-definition cells so readers see clean cells with context
• hard-coded a single cluster (EUCL-Q1-CL-1, z=0.55) and added np.random.seed(45) for reproducible control field selection
• Fixed missing median_filter import and NED column name detection
• Removed unused imports
• Refactored section 8:
• Fixed figure 5 caption and figure 6 zero-match fallback message
• Applied one-sentence-per-line formatting to all markdown cells

closes #290
closes #294