✨ feat: allow relative paths for image shortcodes

[welpo]

Summary

This PR updates all image shortcodes to support relative paths.

Changes

All image shortcodes within the theme have been modified to first attempt to locate images using a relative path. If the image is not found, the shortcode then defaults to the specified absolute path.

Before, for co-located assets, this was necessary (e.g. for a post in blog/shortcodes/index.md):

{{ invertible_image(src="blog/shortcodes/img/graph.webp", alt="Invertible graph") }}

Now, it's possible to use the same format of relative path that you'd use for a normal image:

{{ invertible_image(src="img/graph.webp", alt="Invertible graph") }}

Accessibility

No specific accessibility changes were made in this PR, as the updates primarily concern backend image path handling. The front-end display and accessibility of images remain unchanged.

Type of change

• Bug fix
• New feature
• UI/UX improvement
• Refactor
• Documentation update
• Other (please describe below)

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Checklist

• I have tested all possible scenarios for this change
• I have made corresponding changes to the documentation:
  • Updated "Shortcodes" post in English
  • Updated "Shortcodes" post in Spanish
  • Updated "Shortcodes" post in Catalan