Consider moving theme files to `theme/` subdirectory

## Problem

The repository root is currently cluttered by a mix of theme files and project metadata (editor config, license, code of conduct, etc.).

## Description

Move all theme files to a `theme/` subdirectory. That would make the repository root a bit cleaner and separate metadata from the theme files. Likewise, the `docs/` subdirectory will soon contain theme documentation that we don't want to publish with the theme (#67)

## Alternatives

## Additional context

We would need to understand how this theme is intended to be published and included in the CC website deployments. We would only want to include the contents of `theme/` in the deployed version of this project.

## Implementation

- [x] I would be interested in implementing this feature.