Document(00-intro.md) requires update

[XuJiandong]

https://github.com/ckb-js/lumos/blob/develop/website/docs/00-intro.md

In the document above, there are some out-of-date materials

1. Deprecated settings

predefined.AGGRON4

TESTNET is suggested.

2. Missing import AddressType

const { mnemonic, ExtendedPrivateKey, AddressType } = hd;

3. Simplify URL config

import { RPC, Indexer } from '@ckb-lumos/lumos';

const CKB_RPC_URL = "https://testnet.ckb.dev/rpc";
const CKB_INDEXER_URL = "https://testnet.ckb.dev/indexer";

const rpc = new RPC(CKB_RPC_URL);
const indexer = new Indexer(CKB_INDEXER_URL, CKB_RPC_URL);

The indexer and rpc services are merged. Maybe we can simplify it? An example:

const CKB_RPC_URL = "https://testnet.ckb.dev";
const indexer = new Indexer(CKB_RPC_URL);
const rpc = new RPC(CKB_RPC_URL);

4. typo

    const myMnemonic = mnemonic.generateMnemonic();
    const seed = mnemonic.mnemonicToSeedSync(myMnemonic);
    console.log("my mnemonic: ", myMnemonic); // <-- update this