Document curriculum best practices (e.g., pre-indenting editable regions)

## Description

I'd like to propose adding documentation for curriculum best practices that aren't currently covered. There are important conventions that curriculum contributors should follow, but they're not documented anywhere.

## Example: Pre-indenting Editable Regions

One undocumented best practice is that `--fcc-editable-region--` areas should be pre-indented appropriately for the camper. This ensures:
- The camper's cursor starts at the correct indentation level
- Code written by campers follows proper formatting from the start
- The learning experience feels polished and professional

Without this documentation, contributors may not realize they should set up the editable region with proper indentation, leading to inconsistent experiences across challenges.

## Proposed Solution

Create a "Curriculum Best Practices" guide (or add a section to existing docs) that covers conventions like:
- Pre-indenting editable regions to the appropriate level
- Other formatting standards for seed code
- Common patterns for test assertions
- Guidelines for hint and instruction writing
- Any other tribal knowledge that experienced curriculum contributors know

## Benefits

- Reduces back-and-forth in PR reviews
- Ensures consistency across the curriculum
- Helps new curriculum contributors produce quality work faster
- Documents institutional knowledge that currently only exists in reviewers' heads