This PR adds a page describing how to create, use and moderate a forum.
It creates references in the elearning page as karma features are shared.
It removes redundant information from the helpdesk/overview/help_center
page and links back to the more detailed forum page.
task-3283274
closesodoo/documentation#4245
Signed-off-by: Xavier Platteau (xpl) <xpl@odoo.com>
