[MPOps][Doc] Update link prediction, graph classification 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

- [ ] [Link Prediction using Graph Neural Networks](https://docs.dgl.ai/tutorials/blitz/4_link_predict.html)
- [ ] [5.2 Edge Classification/Regression](https://docs.dgl.ai/guide/training-edge.html)
- [ ] [5.3 Link Prediction](https://docs.dgl.ai/guide/training-link.html)
- [ ] [5.4 Graph Classification](https://docs.dgl.ai/guide/training-graph.html)
- [ ] [6.2 Training GNN for Edge Classification with Neighborhood Sampling](https://docs.dgl.ai/guide/minibatch-edge.html)
- [ ] [6.3 Training GNN for Link Prediction with Neighborhood Sampling](https://docs.dgl.ai/guide/minibatch-link.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