Add a banner to point to https://github.com/WordPress/block-development-examples and archive this repo

[juanmaguitar]

As the new version of "gutenberg-examples" is already in place and examples from this repo have been migrated, I think it's a good time to tackle the following tasks:

• [x] Add a banner to to redirect users to https://github.com/WordPress/block-development-examples
• [x] Archive this repo