docs(nav): reorganize client guides to mirror use-cases structure

Just the Docs builds nav correctly when parent (index.md) and children
are in the same directory. The previous layout had the parent at
docs/multi-client-setup.md and children in docs/client-guides/*.md
(different directory), which caused inconsistent nav rendering across
pages.

Fix: rename docs/multi-client-setup.md to docs/multi-client-setup/index.md
and move all docs/client-guides/*.md into docs/multi-client-setup/.

This exactly mirrors the working use-cases/ structure:
  use-cases/index.md (parent)  + use-cases/*.md (children)  → works
  multi-client-setup/index.md (parent) + multi-client-setup/*.md (children) → fixed"

Author: Dev Agent Amelia