Design new documentation content structure

[peholmst]

We need to gradually refactor and improve our documentation. To make this easier, and to avoid SEO and redirection issues, we need to know the overall content structure up front.

Requirements:

• The Getting Started section should contain everything you need to build a new application from scratch and deploy it to production (the "happy path"). It should be very hands-on, focusing on what you need to do rather than why.
• The Building Apps section should contain everything you need to know to succeed with Vaadin. It should be possible to read it like a book, from start to finish.
• Very technical articles should have their own section, so that they don't interrupt the flow of Building Apps, or confuse new users.
• The documentation covers Vaadin as a platform, not as a set of individual products.
• Different content is aimed at different users. Should we make personas of our audience?

[peholmst]

Actual work on this issue happens in a Google doc.

[peholmst]

Closing this issue as the structure has changed radically from what was listed in this ticket because of #4071 .