clarify considerations between colmena keys vs TF sensitive variables

[KiaraGrouwstra]

as per nix/colmena/options.nix, it looks like colmena keys are exposed in teraflops' machine.deployment.keys.
given terraform/opentofu configuration is also supported, readers may realize another option available would be to use their sensitive variables.
it would be nice to maybe add a bit in the colmena markdown file on this and considerations on their respective use-cases.

[aanderse]

thanks for creating this issue

i will work on a detailed explanation of this so it is entirely clear
an example or two might be a good reference point we well