Add the introduction of emitters for sdk clients.

[lirenhe]

To resolve #5339

Add the introduction to describe the location, the configuration and how to run the client emitters for Java, Python, JS and .NET.

The doc currently only contain the common content and it needs the contribution for all language owners.

cc @lmazuel

[azure-sdk]

No changes needing a change description found.

[azure-sdk]

You can try these changes here

🛝 Playground	🌐 Website	📚 Next docs	🛝 VSCode Extension

[lirenhe]

Need each language owner to update their language specific contents:

Java: @weidongxu-microsoft
JS: @MaryGao
Python: @msyyc
.NET: @JoshLove-msft

[weidongxu-microsoft]

Currently Java emitter does not have important emitter options to add.

We are checking if we need to have a (potentially cross-language) package-name emitter option. E.g. <groupdId>:<artifactId> for Java
(for azure, the <groupId> is com.azure/com.azure.resourcemanager, and <artifactId> is from SDK namespace -- but this is no longer a valid assumption for unbranded)

[lirenhe]

Currently Java emitter does not have important emitter options to add.

We are checking if we need to have a (potentially cross-language) package-name emitter option. E.g. <groupdId>:<artifactId> for Java (for azure, the <groupId> is com.azure/com.azure.resourcemanager, and <artifactId> is from SDK namespace -- but this is no longer a valid assumption for unbranded)

If there is no additional options to add, could you put some information like no additional setting needed for Java under

#### Configuration Options for Java

[lirenhe]

@JoshLove-msft, could you help to update and review the content for .NET? Thanks