Fix og image filenames

[alDuncanson]

This pull request updates the GitHub Action to improve handling of preview image filenames, enhance support for different multi-file modes, and clarify documentation. The main improvements are the dynamic defaulting of the preview image filename to the PDF name when not specified, and the addition of a new "collection" mode for multi-file HTML generation.

Preview image filename handling:

• The output-preview input now defaults to the PDF filename (without extension) if not explicitly set, both in documentation and in the action logic. This ensures more predictable and relevant preview image names. [1] [2] [3] [4] [5]

Multi-file mode enhancements:

• Introduced a new "collection" mode for multi-file HTML generation, where each Markdown file is converted to its own HTML file with associated preview and metadata. This is in addition to the existing "combined" mode.
• Updated step names and conditions to clearly distinguish between "combined" and "collection" modes, improving maintainability and clarity.

Documentation updates:

• Updated the documentation in docs/configuration.md to reflect the new default behavior for output-preview and to clarify the available modes.

[github-actions]

Test Build Preview

Generated from 40bf0b5

Artifacts

Download the generated files from the workflow run:

Single-file mode (test-output-single):

• resume.pdf
• index.html
• resume.png

Combined mode (test-output-combined):

• docs.pdf (combined)
• index.html (auto-generated index)
• docs/*.html (individual pages)
• docs.png

Collection mode - flat (test-output-collection-flat):

• resume.pdf, cover-letter.pdf, references.pdf
• resume.html, cover-letter.html, references.html
• *.png (one per document)

Collection mode - nested (test-output-collection-nested):

• projects/web/.pdf, projects/mobile/.pdf
• projects/web/.html, projects/mobile/.html
• *.png (one per document)