Populate initial user guide docs

[gigony]

Initial user guide implementation.

Instruction to build docs in dev mode:

# Create Python Virtual environment, skip if you already in such environment.
conda create -n monai python=3.6 pytorch torchvision torchaudio cudatoolkit=11.1 -c pytorch -c conda-forge
conda activate monai

# Setup dependent packages
./run setup

# Install jupyter notebook and launch jupyter notebook to update tutorials
pip install jupyter

cd notebooks/tutorials
jupyter notebook .

# Launch another terminal and execute the following commands
# Activate the same Python virtual environment. Skip if you are already in such environment
conda activate monai

# launch documents build system in dev mode
./run gen_docs_dev  # then visit webbrowser to see live docs

# Now you can edit docs by changing files in `notebooks/tutorials/*.ipynb` (in Jupyter notebook) and `docs/source/**/*.md` files.

Reference for Markdown(.md) document

• https://github.com/Project-MONAI/monai-deploy-app-sdk/blob/main/CONTRIBUTING.md#building-the-documentation
• The MyST Syntax Guide
• Optional MyST Syntaxes
• MyST Syntax Reference
• MyST with Sphinx

For Diagram, we use sphinxcontrib-mermaid.

• Mermaid syntax
• We can use classDiagram to visualize Operator(as a class) with multiple input/output labels(input attributes as fields, output attributes as methods) and connections between operators(as an edge label).

You can use the following statements in Jupyter notebook's markdown cell or in .md files.

The workflow of the application would look like this.

```{mermaid}
%%{init: {"theme": "base", "themeVariables": { "fontSize": "16px"}} }%%

classDiagram
    direction LR

    SobelOperator --|> MedianOperator : image...image
    MedianOperator --|> GaussianOperator : image...image

    class SobelOperator {
        <in>image : DISK
        image(out) IN_MEMORY
    }
    class MedianOperator {
        <in>image : IN_MEMORY
        image(out) IN_MEMORY
    }
    class GaussianOperator {
        <in>image : IN_MEMORY
        image(out) DISK
    }
```

Result: