Add gui-init related screenshots and explanations

Nobody has claimed this yet.

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](https://github.com/osresearch/heads/blob/master/initrd/bin/generic-init) usage which is not useful to most boards anymore and replaced with [gui-init](https://github.com/osresearch/heads/blob/master/initrd/bin/gui-init) usage, being [fbwhiptail based](https://github.com/osresearch/heads/blob/master/boards/qemu-coreboot-fbwhiptail/qemu-coreboot-fbwhiptail.config#L28-L30) or [whiptail based, working also over serial connections for servers and user friendly](https://github.com/osresearch/heads/pull/472/files#diff-1ed640a7de731cb1c42290d1a06f2a26R32-R34), 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.

Review the Heads-wiki pages that still describe deprecated generic-init usage, then read the linked Purism documentation and video references alongside gui-init and its framebuffer or serial-based modes. Done means adding relevant screenshots and explanations and updating the documentation to describe gui-init instead of generic-init.

Written by the indexing model from the issue text.