Update Documentation in Conformance to the Documentation Guide

[PSchmiedmayer]

Problem

The current documentation in Spezi provides a good overview of the API and includes documentation for most public APIs. In line with the newly published Stanford Spezi Documentation Guide, we should update the documentation in accordance with the guidelines.

Solution

The documentation should be updated to provide more insightful inline documentation, improve the README file and the DocC landing page in conformance with the Stanford Spezi Documentation Guide.

• Update inline documentation, including links and other elements noted in the Code Documentation section.
• Improve the DocC landing page as detailed in the Landing Page section.
• Add a graphic and visual representation of the Spezi setup and architecture.
• Improve the READMe as noted in the README section.

Additional context

This PR is related to #82

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines