Implement the example gallery

[asinghvi17]

TODOs:

• Make the @cardmeta block rewrite the page to add the cover image directly after the first title element
• Create a constructor for a Card object that populates values from the Dict GALLERY_DICT
• Make this constructor also downsize the cover image by a factor of at least 3, for a lighter page
• Get the example gallery working again!
• Stretch goal: Allow multiple cardmeta blocks on a single page
• Stretch goal: at the end of each page, try to add a default Cover = fig, without the user having to write it...