Update Quark Agent's installation instructions in the next release

Refer to #670.

We currently use the following complex commands to install Quark Agent because Quark Agent is not included in any release.

```
git clone https://github.com/quark-engine/quark-engine.git & cd quark-engine
pip install .[QuarkAgent]
```

In the next release, we should simplify the Quark Agent installation commands as follows.

```
pip install quark-engine[QuarkAgent]
```