Update plugin guide
this MR updates the plugin creation guide as the psyplot-plugin command-line program has been replaced by the psyplot-plugin-template. and it fixes some links in the docs as we now use gitlab.
Edited by Philipp S. Sommer
this MR updates the plugin creation guide as the psyplot-plugin command-line program has been replaced by the psyplot-plugin-template. and it fixes some links in the docs as we now use gitlab.