Create documentation for partners on generating management SDKs using MPG

[ArthurMa1978]

We plan to ship our MPG to customers, allowing them to generate the SDK independently.
Therefore, we need to create documentation that explains how to generate the Azure mgmt. SDK using MPG.
Please refer to the .NET generation documentation for guidance.

[weidongxu-microsoft]

Made a simple guide https://github.com/Azure/azure-sdk-for-java/wiki/Generate-code-from-TypeSpec

Let's see whether we have anyone uses this doc first, before improving the doc.