Website ra3xdh.github.io update

[ra3xdh]

I have updated Qucs-S webpage https://ra3xdh.github.io/ and replaced some outdated screenshots, guides, infos etc. Please check.

[tomhajjar]

For Windows, will people know to create the "C:\Users\xxx.qucs - S" project directory that is created only if you use the Windows installer?

[ra3xdh]

The Windows installer doesn't create any subdirectories in user home directory. It is created on the first application start. The default project directory has changed to $HOME/QucsWorksapce after #145 The Qucs-S doesn't use $HOME/.qucs for projects and cache anymore. Old settings are preserved.