* feat: document plain values
Adds `d.val` to attach type and help information to plain Jsonnet
values, apart from specially treated `fn` and `obj`.
* feat: defaults
Docsonnet packages previously used to be declared in a `main.docsonnet`.
Because this file basically always contains the same, it is kinda
redundant.
To avoid that, the information from there has been inlined into the main
Jsonnet file, while the parser logic will become part of the Go
application, as embedded Jsonnet.
