[MPOps][Doc] Update mini-batch NN module related doc

## 🔨Work Item

**IMPORTANT:**
* This template is only for dev team to track project progress. For feature request or bug report, please use the corresponding issue templates.
* DO NOT create a new work item if the purpose is to fix an existing issue or feature request. We will directly use the issue in the project tracker.

Project tracker: https://github.com/orgs/dmlc/projects/2

## Description

- [ ] [6.5 Implementing Custom GNN Module for Mini-batch Training](https://docs.dgl.ai/guide/minibatch-nn.html)
- [ ] [Stochastic Training of GNN for Link Prediction](https://docs.dgl.ai/tutorials/large/L2_large_link_prediction.html)
- [ ] [Writing GNN Modules for Stochastic GNN Training](https://docs.dgl.ai/tutorials/large/L4_message_passing.html)

**All the changed docs need to have a reference to the doc in the old API style and vice versa.**

## Depending work items or issues