[Doc] Update doc folder README (#5495)

docs/README.md

@@ -4,14 +4,19 @@ DGL document and tutorial folder
 Requirements
 ------------
 You need to build DGL locally first (as described [here](https://docs.dgl.ai/install/index.html#install-from-source)), and ensure the following python packages are installed:
+
 * sphinx==4.2.0
 * sphinx-gallery
 * sphinx_rtd_theme
 * sphinx_copybutton
-* torch
-* mxnet
+* nbsphinx>=0.8.11
+* nbsphinx-link>=1.3.0
 * pillow
 * matplotlib
+* nltk
+* seaborn
+* ogb
+* rdflib
 
 
 Build documents
@@ -21,14 +26,20 @@ First, clean up existing files:
 ./clean.sh
 ```
 
-Then build:
+To build for PyTorch backend only,
 ```
-make html
+make pytorch
+```
+
+To build for MXNet backend only,
+```
+make mxnet
 ```
 
-Note: due to the backend loading issue, it actually takes 2 rounds to build:
-1. build tutorials that uses MXNet as backend
-2. build tutorials that uses PyTorch as backend
+To build for both backends,
+```
+make html
+```
 
 Render locally
 --------------