fix(docs): install mermaid explicitly, was only vocs's optional peer

Mermaid diagrams built and rendered locally but failed in production
with "Could not resolve \"mermaid\" imported by \"vocs\". Is it
installed?" -- mermaid was never an actual dependency, only listed
in vocs's optionalPeers, so it resolved locally from some leftover
install state but was genuinely absent from a clean install matching
what CI/Cloudflare Workers Build actually does. Verified by wiping
node_modules/dist/.vocs and rebuilding from bun install
--frozen-lockfile before committing.

Author: christopherwxyz