docs: Chain Fusion deep dives (#213)

## Summary

- Moves `docs/concepts/chain-fusion.md` →
`docs/concepts/chain-fusion/index.md` (unchanged content, updated
relative links)
- Adds 6 new conceptual sub-pages under `docs/concepts/chain-fusion/`:
- `bitcoin.md` — Bitcoin adapter, Bitcoin canister API, Bitcoin checker
canister (SDN list), ckBTC architecture and flows
- `ethereum.md` — HTTPS outcalls + chain-key ECDSA integration, EVM RPC
canister architecture, ckETH/ckERC20
  - `solana.md` — SOL RPC canister, Ed25519 threshold signing, ckSOL
- `dogecoin.md` — Dogecoin adapter + canister (Bitcoin fork, same
architecture)
- `exchange-rate-canister.md` — XRC on uzr34, request/response format,
cycle costs, median rate logic
- `chain-key-tokens.md` — Architecture (minter/ledger/index/archive),
minting flows, redemption flows, deployed tokens table
- Deletes all 10 staging files from
`.migration/learn-hub/how-does-icp-work/chain-fusion/`
- Updates cross-links in `chain-key-cryptography.md`,
`concepts/index.md`, `choose-your-path.md`, and the four chain-fusion
guide pages

## Sync recommendation

`informed by learn hub staging files — chain-fusion section`

Author: marc0olo