Improve the usability of the user documentation

To improve the user documentation's usability and ease of maintenance, the following changes shall be implemented:

• Highlight current position in the navigation menu
• Unfold the navigation menu to display the current position
• Open the navigation menu only on click (not on hover)
• Extract the footer to a single file which is imported in all pages (just like the navigation menu)
• Update the contact information in the footer

Furthermore, the "AF3 Help" shall no longer use Eclipse's built-in help framework. Instead, a lightweight FX WebView pointing to the online user documentation shall be introduce as a replacement. This will also resolve #4083.