[GIE Doc] Add Tutorials for Gremlin Users (#2369)

<!--
Thanks for your contribution! please review
https://github.com/alibaba/GraphScope/blob/main/CONTRIBUTING.md before
opening an issue.
-->

## What do these changes do?
as titled.
<!-- Please give a short brief about these changes. -->

## Related issue number

<!-- Are there any issues opened that will be resolved by merging this
change? -->

Fixes

Author: shirly121

docs/index.rst

@@ -68,6 +68,7 @@ and the vineyard store that offers efficient in-memory data transfers.
    :caption: Advanced Topics
 
    dive_into_engines
+   supported_gremlin_steps
    storages_and_unified_interface
    performance_tuning