✨ feat(social-media-cards): support relative paths

[welpo]

Description

This PR adds the ability to specify relative paths for the social_media_card in a post. Useful for co-located assets in posts with a structure like:

/content/
    blog/
        post-folder/
            index.md
            social-media-card.jpg
        _index.md

In the example above, the social media card can now be set with social_media_card = social-media-card.jpg instead of its full path.

New Features

• Support for relative paths for social_media_card based on the colocated folder.
• Detailed error messages to aid debugging when the image is not found.

How Has This Been Tested?

• Manually tested the changes by creating posts with relative and absolute paths for the social media cards.
• Verified that appropriate error messages are shown when the image file does not exist at either of the specified paths.

Additional Context

This feature was requested in Issue #161.

[SeaDve]

Thanks for the quick fix!