Implement the example gallery

[asinghvi17]

TODOs:

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