docs(design-proposals): add Kairos tenant worker pools proposal

Kairos worker pools exist as a draft pull request in
cozystack/cozystack without a design proposal. This proposal states
why a second worker OS is worth its cost for tenant pools and makes
a hoster-side immutable NodeGroupClass the whole OS boundary, for
Talos as well as Kairos, so the pool keeps only fields that mean the
same thing on every OS and existing pools move onto Talos classes
without rolling. It folds in the Kairos GPU guest image from the GPU
in Kairos pools proposal (#101), and lists what is carried on top of
the upstream Kairos provider and when it is dropped.

Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <f@lex.la>

Author: lexfrei