docs: add evolution & scaling concept page (fault tolerance, subnet creation, chain evolution)

Migrates 4 Learn Hub articles from the "Evolution & Scaling" section into a
single docs/concepts/evolution-scaling.md page covering fault tolerance and
node recovery, horizontal subnet scaling and XNet messaging, and governance-
driven protocol upgrades via epoch-boundary transitions.

Updates chain-key-cryptography.md: replaces Learn Hub chain-evolution link.
Updates glossary.md: replaces Learn Hub fault-tolerance link.

Author: marc0olo