Broken url for sigpml and marked graph

[dvojtise]

The tutorial page (http://gemoc.org/tutorial.html) refers to the old tutorials : sigPML and Marked graph

They both relates to the MoccML concurrent engine.

• The links are broken. (online versions are still available: gemoc.org/gemoc-studio-old/publish/tutorial_markedgraph/html_single/GuideTutorialMarkedGraph.html and http://gemoc.org/gemoc-studio-old/publish/tutorial_sigpml/html_single/Tutorial_SigPML.html )
• The sources of these tutorials are hidden in an old repo (in commit before switching to Eclipse organisation) (last commit with the sources of these tutorials: https://github.com/gemoc/gemoc-studio-userguide/releases/tag/before_IP_review_cleanup )

Additionally, these tutorials were written for older versions of the Studio and should be updated

[dvojtise]

As the SigPML tutorial relates to the same language as one of the official example, the text of this tuto can probably be upgraded as an official tutorial and/or be used as the language example documentation.