Some updates, code style, headers, color fix

Make headers h3 instead of h2
Make the active menu color the same green as topbar
Fix whitespace consistency
Make it clearer where Overvecht is.
Update main page

Use new main image, fixed area coords.
Updated photos and use same shape as other logo.
Add coloured logo to main page.
Place text on the page, as styled text.
Add some style info to the README