docs: add protocol stack concept pages (Batch 1) (#209)

## Summary
- Creates `docs/concepts/protocol/` subdirectory with 7 new concept
pages: protocol stack overview, peer-to-peer, consensus, message
routing, execution, state synchronization, and performance
- Migrates 6 Learn Hub articles from the "Blockchain Protocol" section
and 1 from the "What is ICP?" section (performance); all Learn Hub links
within these pages replaced with internal paths
- Restores 5 portal diagrams across the protocol pages:
`core_protocol_layers`, `consensus_orders_messages`, `block_maker`,
`consensus_notarization`, `state-sync`; adds 2 performance screenshots
from the Learn Hub
- Performance page: metric values linked to timestamped IC Dashboard API
calls, 12-city RTT matrix table, synthetic benchmark latency data
- Converts Concepts sidebar from `autogenerate` to explicit `items`
array to fix lowercase group labels and control ordering; moves Protocol
Stack after architecture pages; adds Chain Fusion and Protocol Stack
index slugs labeled "Overview" to avoid duplicate group/child labels
- Updates `docs/concepts/index.md`: adds Protocol Stack to Architecture
section
- Updates `docs/concepts/network-overview.md`: trims consensus
subsection (detail now in consensus.md), links to protocol stack
overview

## Staging files deleted
-
`.migration/learn-hub/how-does-icp-work/blockchain-protocol/blockchain-protocol.md`
→ `docs/concepts/protocol/index.md`
-
`.migration/learn-hub/how-does-icp-work/blockchain-protocol/consensus.md`
→ `docs/concepts/protocol/consensus.md`
-
`.migration/learn-hub/how-does-icp-work/blockchain-protocol/peer-to-peer.md`
→ `docs/concepts/protocol/peer-to-peer.md`
-
`.migration/learn-hub/how-does-icp-work/blockchain-protocol/message-routing.md`
→ `docs/concepts/protocol/message-routing.md`
-
`.migration/learn-hub/how-does-icp-work/blockchain-protocol/execution-layer.md`
→ `docs/concepts/protocol/execution.md`
-
`.migration/learn-hub/how-does-icp-work/blockchain-protocol/state-synchronization.md`
→ `docs/concepts/protocol/state-synchronization.md`
- `.migration/learn-hub/out-of-scope/what-is-icp/performance.md` →
`docs/concepts/protocol/performance.md`

## Sync recommendation
hand-written

<!-- Upstream: informed by Learn Hub articles "Blockchain Protocol",
"Consensus", "Peer to peer", "Message Routing", "Execution Layer",
"State Synchronization", "Performance" (migrated, source retired) -->

Author: marc0olo