Blocks: add block examples to each of our blocks

[jeherve]

It would be nice to add Block examples to all our blocks, using the new help panel in the block editor:
WordPress/gutenberg#16813
WordPress/gutenberg#16979

#13565 aims to bring previews for several blocks.

• Business Hours Business Hours block - add example #13609
• Contact Form Contact form block - add example #13629
• Contact Info Contact info block - added example #13635
• Gif Gif block - add example #13610
• Mailchimp Mailchimp block preview #13770
• Map Show an image placeholder for the map block #13731
• Markdown Markdown block - add example #13613
• Recurring Payments Recurring Payments block - added example #13823
• Related Posts Related posts block - add example #13615
• Repeat Visitor Repeat visitor block - add example #13640
• Simple Payments Simple Payments Button block - added example #13579
• Slideshow Slideshow block - add example #13581
• Subscriptions Subscription Form block - block - added example #13821
• Tiled Gallery Tiled Gallery Block - added example #13580
• WordAds Ad block - add preview #13822