Update klaytn tutorials for dApp developer

[kjeom]

Klaytn docs - tutorial update

• [x] https://github.com/klaytn/klaytn-docs/pull/487

• [x] https://github.com/klaytn/klaytn-docs/pull/486

• [x] https://github.com/klaytn/klaytn-docs/pull/488

• [x] update stale data

• [x] move prerequisites to the beginning of the page

• [x] metamask update (with gasprice description)

• [x] add online-toolkit doc