Add section on terminology

There's a lot of jargon used throughout the project. Let's add a section to help clarify some terms. Some examples of this include:
- dependency graph
- manifest files
- organization
- module
- source
- dependency
- dependent

This should be added to the `/docs/concept/` page once gh-53 is merged

Start by reviewing the /docs/concept/ page and checking the status of gh-53, which this work depends on. Add a terminology section covering dependency graph, manifest files, organization, module, source, dependency, and dependent; done means the terms are clearly explained on that page after gh-53 is merged.

Written by the indexing model from the issue text.