Add images for block editor's Welcome Guide

[noisysocks]

Trac ticket: TBD

Adds the image assets necessary for WordPress/gutenberg#32026.

To test:

1. Clone the wordpress-develop repo, check out Add images for block editor's Welcome Guide #1269, build using npm i; grunt build.
2. Clone the gutenberg repo, check out Update Welcome Guides to use external image URLs gutenberg#32026, build using npm i; npm run build.
3. Configure your environment to run both local copies. For example, if using wp-env, modify gutenberg/.wp-env.json to set "core": "/path/to/wordpress-develop/build", then run wp-env start.
4. Go to Posts → Add New, open the Options menu, and click Welcome Guide. The guide's images should appear normally.
5. Go to Appearance → Widgets, open the Options menu, and click Welcome Guide. The guide's images should appear normally.
6. Go to Appearance → Customize → Widgets, open the Options menu, and click Welcome Guide. The guide's image should appear normally.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.