Highlight in balance data related chapters on how to enable this feature

[wey-gu]

Change Request

This repository is ONLY used to solve issues related to DOCS.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

User queried this on how the feature is still not usable in 3.1

2. Describe your suggestion or addition.

We could add notices to make them aware of how it's not turned on by default and where to switch it on.

3. Provide some reference materials (documents, websites, etc) if you could.

[cooper-lzy]

The Balance Data feature is supported in the enterprise edition.

[wey-gu]

For whom found this issue being confused, enable_experimental_feature is the flag to enable BALANCE DATA in Nebula Graph version of 3.1.0 +