[WIP] build pkgs documentation with DemoCards

[johnnychen94]

This is still a WIP PR; it requires the master branch of DemoCards.

The idea is to move docs/src/pkgs to docs/pkgs and build it with DemoCards, this enables us to collect remote documentation in sub packages (e.g., ImageFiltering), and furthermore, we could gradually move the current pkgs docs back to their repo.

This is the new folder structure, note that docs/pkgs/config.json is added for order specification:

docs
├── examples
│   ├── color_channels
│   ├── config.json
│   ├── contours
│   ├── image_quality_and_benchmarks
│   ├── index.md
│   └── spatial_transformation
├── make.jl
├── pkgs
│   ├── axes
│   ├── config.json
│   ├── features
│   ├── introduction
│   ├── metadata
│   ├── segmentation
│   └── transformations
└── src
    ├── api_comparison.md
    ├── assets
    ├── function_reference.md
    ├── index.md
    ├── install.md
    └── tutorials

[ashwani-rathee]

I think I could help with this,I think it needs a lot of rewriting and changing but democards.jl will be much better option.How can I help?

[johnnychen94]

I still don't have a clean and clear view of what changes we should do here until I finished JuliaDocs/DemoCards.jl#72