Merge: Improve documentation of kernel

Merge: Improve documentation of kernel

Improve the documentation of Object and Sys.
Also experiments with the new nitdoc features #1447

Some issues I discovered while writing and using the doc while thinking about nitdoc.

* micro-picking classes and methods to include in the README is not fun
* headings (`###`) in the markdown might not have the right level in the generated HTML
* having reified property groups (#1413) should help to organize thing and have automatic feedback on it trough the doc
* maybe the `[[list:]]` directive should no list entities not already included by `[[doc:]]` (or a new `[[list_other:]]` directive?)
* having a way to generate a full linear and strucuted documentation in a single page could simplify the job of the writer when checking the doc.

Pull-Request: #1584
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>