Fixing broken links on usb.rst
Fixing broken links on gpio.rst
Fixing broken PDF link on external-interrupts.rst
Fixed broken links in i2c.rst

I found the correct redirect for the Arduino page, but couldn't find the equivalent ST PDF. I think I found the closest thing but please advise.
Fixed bad links for pwm.rst, spi.rst, usart.rst
fix broken link on maple native beta page
fix broken link to usb.org 2.0 spec
fix codesourcery.com broken link
fix wikipedia.org broken link
Explicit Windows XP non-support

Also, note that more recent OS X probably works.