'generic weirs' in the telemac2d user documentation

addition of a subsection 'option 2 : generic weirs' in the telemac2d user documentation

added area:documentation module:telemac-2d labels

created branch 1043-generic-weirs-in-the-telemac2d-user-documentation to address this issue

mentioned in merge request !209 (merged)

closed with merge request !209 (merged)

mentioned in commit acc3a0e2