Summary

This PR fixes issue #338 where embedded/built-in shortcodes were not being loaded when a site didn't have a shortcodes directory.

Problem

When users ran marmite myblog --shortcodes on a site without a shortcodes directory, the list of available shortcodes was empty. Additionally, embedded shortcodes like <!-- .youtube id=xyz --> were not being processed during site generation.

Solution

• Modified ShortcodeProcessor::collect_shortcodes() to always load built-in/embedded shortcodes first, before checking if the user's shortcodes directory exists
• Fixed type mismatch when passing shortcode processor reference to handle_content_pages()
• Updated documentation to clarify that built-in shortcodes are always available

Testing

Tested with the reproducer from issue #338:

$ marmite myblog --init-site
$ marmite myblog --shortcodes
# Now correctly shows all embedded shortcodes

$ echo '# test\n This is a youtube video\n <!-- .youtube id=xyz -->' > myblog/content/test.md
$ marmite myblog
# YouTube shortcode is now properly processed and generates iframe

Fixes #338