graphql reference documentation page

## Motivation  

[Why is this Task important? Explain the context and reasons for this task.]  

- Our project is open-sourced and it should be easy for users to understand how to use it without looking at the code.

## Objective  

[Clearly state the purpose or goal of this task.]  

- [https://docs.backend.ai/en/latest/manager/rest-reference/index.html](https://docs.backend.ai/en/latest/manager/rest-reference/index.html)
Currently, the REST API provides API reference documentation in the form above.
Let's make graphql provide the same reference documentation.

## Details  

[Outline the specific steps or changes needed to complete this task.]  

1. Select the appropriate static documentation delivery tool for graphql.
1. Add an index.rst that adds a graphql-reference page to your docs.

JIRA Issue: BA-447