docs: clarify seed code indentation and hyphenation rules

[majestic-owl448]

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• I have read and followed the contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Updates the "Formatting Seed Code" section in the coding challenges guide:

• Clarifies that HTML, CSS, and JavaScript use 2-space indentation
• Adds that Python uses 4-space indentation (PEP 8 standard) and single quotes
• Updates the front-end/back-end hyphenation rule to match the writing style guide (hyphenated form)