Improve view docs with examples

As Tryton interface is really restricted to the format, a good improvement could be to add to Views section, some visuals, can be in table console format or embeding img, to use as a cheatsheet for common examples views (groups, colspan, cols, contexts) but also with some tricks like: <group colspan="2" col="-1" id="buttons">.

That way a newbie could quickly see the max power we have in the interface and serve also for us as developers to quickly saw all the options to design good interfaces.

As long as such visual can not be generated automatically (by the build process), it will not happen.