Add gui-init related screenshots and explanations

[tlaurion]

Should we take parts from Purism documentation and videos?

• [ ] https://wp.puri.sm/posts/the-librem-key-makes-tamper-detection-easy/
• [ ] https://docs.puri.sm/PureBoot.html

Heads-wiki documentation is still linked to deprecated generic-init usage which is not useful to most boards anymore and replaced with gui-init usage, being fbwhiptail based or whiptail based, working also over serial connections for servers and user friendly, using the same gui-init to render over framebuffer or simple tty, which was more tested and user friendlier then generic-init.

Linked to https://github.com/osresearch/heads/issues/477 (I think we can close, actually.)

Help wanted.

[tlaurion]

Subpart of #25