Add Pangolin and Serval to Open Firmware overview

[PanderMusubi]

Please, add Pangolin and Serval to Open Firmware overview that is shown at https://support.system76.com/articles/open-firmware-systems/

See also:

• https://github.com/system76/docs/blob/master/content/open-firmware-systems.md#hardware-utilizing-open-firmwareopen-ec
• https://system76.com/laptops/pang15/configure
• https://system76.com/laptops/serw13/configure

[ahoneybun]

The Serval does need to be there (with a note about the bonw15-b) but the Pangolin does not have Open Firmware or Open EC.

[ahoneybun]

Ah I see, fixing this in PR: https://github.com/system76/docs/pull/1263