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>

diff --cc lib/standard/kernel.nit
Simple merge

diff --cc tests/sav/error_class_glob.res
index 87aeaec,b95b280..a1946e2
--- 1/tests/sav/error_class_glob.res
--- 2/tests/sav/error_class_glob.res
+++ b/tests/sav/error_class_glob.res
@@@ -1,12 -1,12 +1,12 @@@
- ../lib/standard/kernel.nit:29,1--100,3: Error: `kernel#Object` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:102,1--116,3: Error: `kernel#Sys` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:129,1--187,3: Error: `kernel#Comparable` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:189,1--226,3: Error: `kernel#Discrete` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:228,1--245,3: Error: `kernel#Cloneable` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:247,1--302,3: Error: `kernel#Numeric` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:308,1--331,3: Error: `kernel#Bool` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:333,1--415,3: Error: `kernel#Float` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:417,1--519,3: Error: `kernel#Byte` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:521,1--712,3: Error: `kernel#Int` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:714,1--867,3: Error: `kernel#Char` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
- ../lib/standard/kernel.nit:869,1--876,3: Error: `kernel#Pointer` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:32,1--225,3: Error: `kernel#Object` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:227,1--300,3: Error: `kernel#Sys` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:313,1--371,3: Error: `kernel#Comparable` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:373,1--410,3: Error: `kernel#Discrete` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:412,1--429,3: Error: `kernel#Cloneable` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:431,1--486,3: Error: `kernel#Numeric` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:492,1--515,3: Error: `kernel#Bool` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:517,1--599,3: Error: `kernel#Float` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:601,1--703,3: Error: `kernel#Byte` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
+ ../lib/standard/kernel.nit:705,1--896,3: Error: `kernel#Int` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
 -../lib/standard/kernel.nit:898,1--1038,3: Error: `kernel#Char` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
 -../lib/standard/kernel.nit:1040,1--1047,3: Error: `kernel#Pointer` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
++../lib/standard/kernel.nit:898,1--1051,3: Error: `kernel#Char` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?
++../lib/standard/kernel.nit:1053,1--1060,3: Error: `kernel#Pointer` does not specialize `module_0#Object`. Possible duplication of the root class `Object`?