[Feature]: Enhance text embossing with grayscale height mapping

# Description

Currently, the text rendering in `gh-skyline` uses a binary threshold for pixel values when creating the 3D text geometry. This results in a "hard" embossing effect where pixels are either fully raised or not at all. We can improve the visual quality by using the pixel's grayscale/whiteness value to determine the embossing height, which would preserve font anti-aliasing in the 3D output.

# Requirements

- Modify the text rendering logic to use pixel whiteness values (0-1) to scale embossing height
- Update `isPixelActive()` to return a float value representing pixel intensity instead of boolean
- Scale the voxel depth based on the pixel intensity value
- Maintain backward compatibility with existing cube generation functions
- Handle edge cases (completely black/white pixels) appropriately

# Definition of Done

- [x] Text rendering uses grayscale values to determine embossing height
- [ ] Anti-aliased edges of text are preserved in the 3D output
- [x] Documentation is updated to reflect the new behavior
- [x] Unit tests cover various pixel intensity scenarios
- [ ] Visual regression tests show improved quality at text edges

# Additional Notes

- Consider adding a configuration option to toggle between binary and grayscale modes
- Pixel intensity calculation should account for RGB channels appropriately
- May need to adjust overall embossing depth to maintain visual balance
- Consider impact on STL file size due to increased geometry complexity