Provide a specification for dfx.json

[nmattia]

Was: #1963

It would be great to have a specification for dfx.json. There is a schema available, but I am not sure how to read it (and is a bit hard to navigate).

Here are some examples of good references for config files:

• npm's package.json
• cargo's Cargo.toml
• GitHub Actions workflows

[sesi200]

This is already an internal ticket: SDK-699