Adding walkthrough from the manuscript to online doc

[yibeichan]

Hi, I added our manuscript's "practice walkthrough" section to this online document. It is structured as the following (starting from User Guide:):

• Welcome: "index.md"
• Introduction: "01_introduction.md"
• Project structure: "20_project_structure.md"
• Schema: "30_schema.md"
• User Guide:
  • Create a research protocol: "41_create_new_protocol.md"
  • Adopt assessments from the library: "42_adopt_assessments.md"
  • Create new assessments for a protocol: "43_create_new_assess.md"
  • Add a feedback section: "44_setup_feedback.md"
  • Finalize the protocol: "45_finalize_protocol.md"
  • Additional tools: "46_tools.md"

[yibeichan]

I finished this online doc, and it is ready to be merged. Some links might be updated once we merge PRs from reproschema-py this one and this one