There's no clear documentation on how to read request data

[peruibeloko]

This issue is mostly being opened to track #6563, which I provided some improvements to the API section of the docs. From the PR:

Didn't find any information on this topic in the entire Phoenix documentation (nor in Plug's), most StackOverflow questions related to this topic are from either 11 years ago and refer to Plug's read_body function or are referring to reading raw request data, which isn't my case.

This PR documents:

• How to access the request's body, path parameters and query parameters.
• The very unintuitive behavior of "data merging" (which might actually be a bug)
• The command with all --no-* flags for creating a bare bones JSON API.