Add documentation for the registry #4717

Where
I think the actual page should probably go under the configure or modify section in the blockly developer guides
.
Some places that it might be worth adding links to the new registry documentation:

• In the interface documentation since I think this is one of the places we expect developers to make use of our interfaces.
• Under forking since when possible we would rather people override certain classes instead of forking blockly or monkey
  patching.

What

• Text
• Image or Gif
• Other

Old content

Suggested content
Information about Blockly.registry.register and how to add the registered item to the plugins object in the inject object.

Additional context