Add liquidstaking2.0 design and methods to gitbook

[mclyk]

Motivation

It's better to write a document on Gitbook page, https://docs.parallel.fi/ Let's do it while finishing the code.

Suggested Solution

• [x] add overview and architecture process
• [x] add data storage
• [x] add methods
• [x] add event and errors

[mclyk]

Missing claim/match part, the other is finished. https://docs.parallel.fi/dev/staking-v2/liquid-staking