Better documenting how to create manifest-based data plugin

[newgene]

• Find out all possible options for the manifest.json file from the code, and create a full example json file, or if needed, can be multiple example json files (e.g., one for simple uploader, another one for multi uploaders)
• We may need a dedicated "data plugin" doc page under /docs folder (also hosted at docs.biothings.io too) to explain everything about "data plugin".