[INFRA] Add a job which regenerate automatically the documentation (.Rd) on each PR

[csouchet]

Add a new job who execute the command devtools::document(), and commit the change in the current PR if there is updating in the documentation.

See also #30

[tbouffard]

⚠️ our need is to ensure that the generated documentation is always up to date. A job may be a way to do it, but this is not mandatory. I suggest we rename the issue to match what we need.