fix: broken PNG image paths in GitHub Pages documentation

[krystophny]

Problem

The PNG images in the GitHub Pages documentation are broken and not displaying. This is a critical defect affecting the usability of the documentation.

Symptoms

• Documentation pages reference images with relative paths like ../../media/examples/basic_plots/simple_plot.png
• These images exist locally at /doc/media/examples/ but are not accessible on GitHub Pages
• Example broken link: https://lazy-fortran.github.io/fortplot/page/example/basic_plots.html shows broken image references

Root Cause Analysis

The GitHub Actions workflow (docs.yml) copies media files to build/doc/media/examples/ after FORD generates the documentation. However, FORD generates HTML files in subdirectories that expect the media files at different relative paths.

The issue appears in these locations:

1. docs.yml lines 84-85: Media files are copied to build/doc/media/examples/ AFTER FORD runs
2. Makefile lines 95-108: Similar copy operation for local builds
3. Example markdown files: Use relative paths like ../../media/examples/ which don't resolve correctly in the deployed structure

Impact

• All example documentation pages have broken images
• Users cannot see the visual output of the examples
• Documentation is significantly less useful without the visual demonstrations

Acceptance Criteria

• PNG images display correctly on all example pages
• PDF download links work correctly
• Media files are accessible at the correct paths on GitHub Pages
• Both local documentation builds and GitHub Pages deployment work correctly

Technical Details

The media files need to be either:

1. Copied to the correct location before FORD processes the markdown files, OR
2. The relative paths in the markdown files need to be adjusted to match the actual deployment structure, OR
3. FORD configuration needs to be adjusted to handle media files correctly

This is a defect, not a feature request - the functionality was intended to work but is broken.