Add more examples + a basic metadata framework

[asinghvi17]

https://geo.makie.org/previews/PR233
The metadata framework doesn't apply to anything yet, but it should do so quite soon! I will have to guard against garbage collection somehow, though. Maybe by setting the value of cover to the relevant module?

The actual implementation will come in the next PR.

[asinghvi17]

@lazarusA you might be interested in examples/basic.jl and docs/example_meta_block.jl here - they have a first prototype for in-code evaluation of an example's metadata (title, description, cover) which we can forward to the Gallery prototype here. (Just have to figure out how to embed/downsize the image now :D)

[asinghvi17]

TODOs for the next PR:

• 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!
• 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...