add Mullvad case study

[orangecms]

https://github.com/mullvad/system-transparency

stboot is System Transparency Project’s official bootloader. It is a LinuxBoot distribution based on u-root. The source code of stboot can be found at https://github.com/system-transparency/stboot.

[tlaurion]

Should point to https://git.glasklar.is/system-transparency/core/stboot