Describe verified boot

[mchack-work]

Description

Describe verified boot in Castor version.

Type of change

• Documentation (a change to documentation)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes

[mchack-work]

We should maybe simplify the description in Bellatrix intro as well by using the first parapgraphs, excepting the domain stuff, from this change?