[Docs] Add English Colab tutorials. (#497)

* Add English-version Colab tutorials

* Revise Chinese-version Colab tutorials.

* Add colab tutorials menu on readthedocs page.

* Fix lint

docs/conf.py

@@ -85,6 +85,28 @@ html_theme_options = {
             'name': 'GitHub',
             'url': 'https://github.com/open-mmlab/mmclassification'
         },
+        {
+            'name':
+            'Colab Tutorials',
+            'children': [
+                {
+                    'name':
+                    'Train and inference with shell commands',
+                    'url':
+                    'https://colab.research.google.com/github/'
+                    'open-mmlab/mmclassification/blob/master/docs/tutorials/'
+                    'MMClassification_tools.ipynb',
+                },
+                {
+                    'name':
+                    'Train and inference with Python APIs',
+                    'url':
+                    'https://colab.research.google.com/github/'
+                    'open-mmlab/mmclassification/blob/master/docs/tutorials/'
+                    'MMClassification_python.ipynb',
+                },
+            ]
+        },
         {
             'name':
             'Projects',

docs_zh-CN/conf.py

@@ -85,6 +85,28 @@ html_theme_options = {
             'name': 'GitHub',
             'url': 'https://github.com/open-mmlab/mmclassification'
         },
+        {
+            'name':
+            'Colab 教程',
+            'children': [
+                {
+                    'name':
+                    '用命令行工具训练和推理',
+                    'url':
+                    'https://colab.research.google.com/github/'
+                    'open-mmlab/mmclassification/blob/master/docs_zh-CN/'
+                    'tutorials/MMClassification_tools_cn.ipynb',
+                },
+                {
+                    'name':
+                    '用 Python API 训练和推理',
+                    'url':
+                    'https://colab.research.google.com/github/'
+                    'open-mmlab/mmclassification/blob/master/docs_zh-CN/'
+                    'tutorials/MMClassification_python_cn.ipynb',
+                },
+            ]
+        },
         {
             'name':
             '算法库',