Improve developer documentation for creating plugin blocks

[nylen]

Follow-up tasks to #1619 and #1786.

• Transfer https://github.com/nylen/gutenberg-examples to https://github.com/WordPress/gutenberg-examples
• Add a short readme to the above examples repo
• Review http://gutenberg-devdoc.surge.sh/ and add links to relevant sections of the examples repo
• Remove or dramatically simplify blocks/README.md with links to the above documentation

[lumberman]

@nylen thanks for great code examples!
By any change you can create a simple example on how to use image/upload feature in the custom block? I was trying to follow the default image module, but for some reason it doesn't work. Maybe there is a way to reuse default blocks like Image/Text/Button in our custom blocks? It will be great to not copy/paste functionality that is already in place.

[mtias]

Closing as outdated. The handbook with some tutorials now lives at: https://wordpress.org/gutenberg/handbook/blocks/writing-your-first-block-type/