update user guide

[musslick]

An issue to update the user guide for the documentation.

Things to do:

• Add page to user guide within autora-core under "Workflow" that explains Standard State and variables of the Standard State
• Refer to the separate documentation page of the Standard State and Variables in the "Implementation Sections" of the following Modules Docs: Experimentalist,
• Refer to the separate documentation page of the Standard State and Variables in the "Implementation Sections" of the following Modules Docs: Experiment_Runner (theorist should be fine)