docs: Bitcoin integration guide (#41)

## Summary
- Two-approach structure: ckBTC (recommended) and Direct Bitcoin API
(advanced)
- ckBTC integration: deposit flow, transfers, withdrawals with
Motoko/Rust tabbed code examples
- CLI examples for interacting with ckBTC canisters
- Common mistakes section (6 pitfalls)
- Direct Bitcoin API: available endpoints, `get_blockchain_info` example
using the new `ic-cdk-bitcoin-canister` crate
- Cycle costs table including per-byte costs for
`bitcoin_send_transaction` (8M/20M cycles per byte)
- Development setup: bitcoin-starter template, local regtest with
Docker, multi-environment icp.yaml
- Deploy to testnet and mainnet

## Sync recommendation
Informed by `dfinity/portal` — `docs/build-on-btc/*`,
`docs/references/bitcoin-how-it-works.mdx`,
`docs/references/cycles-cost-formulas.mdx`; `dfinity/icskills` —
`skills/ckbtc/SKILL.md`; `dfinity/icp-cli-templates` —
`bitcoin-starter/`; `dfinity/cdk-rs` — `ic-cdk-bitcoin-canister 0.2`

Author: marc0olo