Improve menu structure

@akwizgran @grote

Referencing a chat on the UX channel:

i think submenus would be fine (assuming we do them in css). we might want to do some user testing to make sure people with different amounts of technical skill can find what they're looking for

about, documentation and download are pretty common top-level menu items for software projects, although i agree download may not be necessary for us as it's on the front page

so perhaps:

about
    news
    team
    contact
documentation
    how it works
    user manual
    faq
    wiki
community
    get involved
    source code
    style guide

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I don't see the point of having submenus. We can show only the most important stuff by keeping the menu structure as simple as:

• About
• Download
• Documentation
• Support
• News
• Contribute
• Donate