Unify/factorize plugin desscription with sources

[bakpaul]

Scripts to auto generate the two pages :

• https://sofa-framework.github.io/doc/getting-started/build/activate-plugins/
• https://sofa-framework.github.io/doc/plugins/suported-plugins-list/

Plugins (and only them) have got a method in the init called getModuleDescription that should

1. Return the same string as the one in the doc (present in the files here) to unify the descriptions.
2. Might be used to get the description string while generating the files (to factorize)

The first point is straightforward. I don't feel the second one is really wanted because it will introduce a different mechanism for a subpart of the modules (aka only the plugins). So it will only make the scripts more complex and more easily breakable by changes in the sources of SOFA.

WARNING the description should not contain line break, because it might break the table in markdown.