#5576329 Adding Universal Nav slide to ETK Whats New

Changes proposed in this Pull Request

• This PR adds a new slide highlighting the Universal nav rollout
• Context in pbAPfg-1ry-p2

Screenshot of the new slide:

Testing instructions

• Checkout this PR and run yarn install.
• sandbox a test site (update /etc/hosts to point the domain for one of your test Simple sites to your sandbox; for example, this entry for one of my sites 192.0.93.101 dsfdsf3.wordpress.com.
• Install Composer (run composer install from Calypso root.
• Sync ETK to your sandbox ( cd into CALYPSOROOT/apps/editing-toolkit and run yarn && yarn dev --sync from the ETK root)
• yarn start Calypso
• Navigate to your sandboxed test site
• Go to the Gutenberg editor and launch the What’s New modal from the sidebar nav. (http://calypso.localhost:3000/post/SITESLUG)
• The What’s New modal is accessed via the small hamburger icon next to the Jetpack logo. Scroll down to the Tools section of the menu and click What's New
• Click through the slides and confirm that they work as expected.