Get value from a website using Yún and a RPi
You need to use the Process library that comes with the IDE.

That allows you to execute commands on the Linux portion of the Yun from within your Arduino code, and get data back from it.

You use that class to execute the curl command in Linux to fetch the content of a web page and return that content to the AVR code where you can deal with it.

If the content of the web page is complex it may be worth passing the output of curl through some other Linux based commands first (such as grep and cut or awk) to clean it up and reduce the amount of text you have to deal with at the AVR end.