Move plugin.mandatory to installing plugins docs
Move plugin.mandatory to installing plugins docs

This commit moves the plugin.mandatory settings from the plugin
directory page in the docs to the installing plugins page in the docs.

strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra