docs: Make SEEK_SDK_OVERRIDE more consistent across documentation and…

[AlexKaravaev]

When setting up repo https://github.com/worldcoin/orb-software/blob/main/CONTRIBUTING.md tells you to set SEEK_SDK_OVERRIDE in .envrc.example in which VENDORED_SEEK_PATH is used.

Also seek-camera will ignore both VENDORED_SEEK_PATH/SEEK_SDK_OVERRIDE.

Not a big change but improves onboarding process a bit.