Extend rescue mod documentation with medkit signature description
Documentation is describing how to verify sha256 checksum but signature verification is missing. see https://docs.turris.cz/hw/omnia/rescue_modes/#re-flash-router
We should also describe what the signature algorithm is used.
This should help meet the following mandatory requirements for Tier 1 defined by CSA Singapore's Cybersecurity Labeling Scheme
- 5.3-2:When the device is not a constrained device, it shall have an update mechanism for the secure installation of updates.