move docs under en_US (#744)

* move en docs under en_US

* update gitignore

* fix image link

* add doc guide

* add napoleon ext

docs/.gitignore

-_build
-_static
+*/_build
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-_templates
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docs/CONTRIBUTING.md → docs/en_US/CONTRIBUTING.md

@@ -38,4 +38,14 @@ Head over to [issues](https://github.com/Microsoft/nni/issues) to find issues wh
 A person looking to contribute can take up an issue by claiming it as a comment/assign their Github ID to it. In case there is no PR or update in progress for a week on the said issue, then the issue reopens for anyone to take up again. We need to consider high priority issues/regressions where response time must be a day or so. 
 
 ## Code Styles & Naming Conventions  
-We follow [PEP8](https://www.python.org/dev/peps/pep-0008/) for Python code and naming conventions, do try to adhere to the same when making a pull request or making a change. One can also take the help of linters such as `flake8` or `pylint`
+* We follow [PEP8](https://www.python.org/dev/peps/pep-0008/) for Python code and naming conventions, do try to adhere to the same when making a pull request or making a change. One can also take the help of linters such as `flake8` or `pylint`
+* We also follow [NumPy Docstring Style](https://www.sphinx-doc.org/en/master/usage/extensions/example_numpy.html#example-numpy) for Python Docstring Conventions. During the [documentation building](CONTRIBUTING.md#documentation), we use [sphinx.ext.napoleon](https://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html) to generate Python API documentation from Docstring.
+
+## Documentation 
+Our documentation is built with [sphinx](http://sphinx-doc.org/), supporting [Markdown](https://guides.github.com/features/mastering-markdown/) and [reStructuredText](http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html) format. All our documentations are placed under [docs/en_US](https://github.com/Microsoft/nni/tree/master/docs).
+
+* Before submitting the documentation change, please __build homepage locally__: `cd docs/en_US && make html`, then you can see all the built documentation webpage under the folder `docs/en_US/_build/html`. It's also highly recommended taking care of __every WARNING__ during the build, which is very likely the signal of a __deadlink__ and other annoying issues.
+
+* For links, please consider using __relative paths__ first. However, if the documentation is written in Markdown format, and:
+    * It's an image link which needs to be formatted with embedded html grammar, please use global URL like `https://user-images.githubusercontent.com/44491713/51381727-e3d0f780-1b4f-11e9-96ab-d26b9198ba65.png`, which can be automatically generated by dragging picture onto [Github Issue](https://github.com/Microsoft/nni/issues/new) Box.
+    * It cannot be re-formatted by sphinx, such as source code, please use its global URL. For source code that links to our github repo, please use URLs rooted at `https://github.com/Microsoft/nni/tree/master/` ([mnist.py](https://github.com/Microsoft/nni/blob/master/examples/trials/mnist/mnist.py) for example).
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docs/HowToDebug.md → docs/en_US/HowToDebug.md

-**How to Debug in NNI**
-===
-
-*Coming soon*
+**How to Debug in NNI**
+===
+
+*Coming soon*

docs/KubeflowMode.md → docs/en_US/KubeflowMode.md

@@ -23,7 +23,7 @@ Now NNI supports running experiment on [Kubeflow](https://github.com/kubeflow/ku
 5. To access Azure storage service, NNI need the access key of the storage account, and NNI use [Azure Key Vault](https://azure.microsoft.com/en-us/services/key-vault/) Service to protect your private key. Set up Azure Key Vault Service, add a secret to Key Vault to store the access key of Azure storage account. Follow this [guideline](https://docs.microsoft.com/en-us/azure/key-vault/quick-create-cli) to store the access key.
 
 ## Design 
-![](./img/kubeflow_training_design.png)
+![](../img/kubeflow_training_design.png)
 Kubeflow training service instantiates a kubernetes rest client to interact with your K8s cluster's API server. 
 
 For each trial, we will upload all the files in your local codeDir path (configured in nni_config.yml) together with NNI generated files like parameter.cfg into a storage volumn. Right now we support two kinds of storage volumns: [nfs](https://en.wikipedia.org/wiki/Network_File_System) and [azure file storage](https://azure.microsoft.com/en-us/services/storage/files/), you should configure the storage volumn in NNI config YAML file. After files are prepared, Kubeflow training service will call K8S rest API to create kubeflow jobs ([tf-operator](https://github.com/kubeflow/tf-operator) job or [pytorch-operator](https://github.com/kubeflow/pytorch-operator) job) in K8S, and mount your storage volumn into the job's pod. Output files of kubeflow job, like stdout, stderr, trial.log or model files, will also be copied back to the storage volumn. NNI will show the storage volumn's URL for each trial in WebUI, to allow user browse the log files and job's output files.