Update model documentation

[cdboer]

Update the existing provenance model documentation in /docs to reflect the changes done during github2prov development.

• [x] Update node attributes
• [x] Update list of system notes / github resource events
• [x] Update node names
• [ ] GitHub2PROV specific explanations

[cdboer]

Maybe we should change the document structure to better account for GitHub/GitLab duality.