nitdoc: factorize more services into `MEntity`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: better display of concerns

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: better display of section headers

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: generate a documentation page for all MGroup

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: generate a documentation page for all mproperties

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: auto hide source-link

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>