Restructure docs.

[dbrumann]

• Splits up README.md into multiple documents
• Adds Contribution Guide explaining the how to run tests locally
• Updates parts of README.md

Probably best reviewed by looking at the resulting docs on my branch: https://github.com/dbrumann/deptrac/tree/restructure_docs

Rationale

The README.md file has become quite large. The table of contents helps with finding sections, but I would like keep the file short with only the necessities for getting started and then provide more in depth documentation in the docs/ folder.

Open To Dos

• Possibly move parts the section Run deptrac in the README.md into docs/ (e.g. a separate page for depfile where parameters and sections are fully explained or an overview of all available commands)
• Add a table of contents to pages in docs
• Add links between pages in docs for easier navigation?