Update model signing documentation (#484)

Graduate one maintainer to emeritus status, add GitHub handles to all Maintainers, add links to remaining TODO items in the table at the end (and remove the rows that would still be empty)

Signed-off-by: Mihai Maruseac <mihaimaruseac@google.com>

Author: mihaimaruseac

process/project-lifecycle-documents/model_signing_sandbox_stage.md

@@ -4,10 +4,15 @@
 
 The project has 4 maintainers from 3 different organizations:
 
-*  Laurent Simon, Google, @laurentsimon
-*  Daniel Major, NVidia,
-*  Eoin Wickens, HiddenLayer, @EWickens
 *  Mihai Maruseac, Google, @mihaimaruseac
+*  Daniel Major, NVidia, @major-security
+*  Eoin Wickens, HiddenLayer, @EWickens
+
+#### Emeritus maintainers
+
+We recognize the efforts of maintainers that have started the project but have since moved on towards other projects.
+
+*  Laurent Simon, Google, @laurentsimon
 
 ### Mission of the project
 
@@ -96,10 +101,8 @@ The project should provide a list of existing resources with links to the reposi
 
 | Reference          | URL |
 |--------------------|-----|
-| Main Repository    | TODO |
+| Main Repository    | https://github.com/sigstore/model-transparency |
 | Meeting Agenda     | https://docs.google.com/document/d/18oAsfhfKJurH-YTUFe520CAZS3lkORX1WnZmBv4Llkc/edit#heading=h.etrsjlz02gla |
-| Contributing guide | TODO |
-| Security.md        | TODO |
-| Roadmap            | TODO |
-| Demos              | TODO |
-| Other              | TODO |
+| Contributing guide | https://github.com/sigstore/model-transparency/blob/main/CONTRIBUTING.md |
+| Security.md        | https://github.com/sigstore/model-transparency?tab=security-ov-file |
+| Demos              | https://github.com/sigstore/model-transparency/blob/main/docs/demo.ipynb |