Reference the EAP on the Declarative Gradle site

[oleg-nenashev]

A bulk update to refresh the site a bit.

• [x] Synchronize the theme with community.gradlle.org
• [x] Add EAP announcement to the landing
• [x] Add a page on sharing feedback, embed the form
• [x] Add Google Tag Manager to track visitor experiences

As discussed on Slack, I am not touching the substantial content for now

[oleg-nenashev]

Thanks Tom!