Main menu enhancement

@MaximeJ suggested improving the main menu of the Orekit web site by replacing the "Developers" entry with two entries: "Users" and "Developers".

The first would provide more detailed access to the documentation and to the download page:

• Documentation
  • Javadoc
  • Maven static site
  • Tutorials
• Download

The second would provide access to resources dedicated to developers:

• Forge
• CI
• Contributing