Add data theme definitions table to /guides

- Added `theme-definitions.json` with structured descriptions of all
  data themes (based on the [Definitions Proposal](https://lf-overturemaps.atlassian.net/wiki/spaces/PROJ/pages/353927169/Definitions+Proposal))
- Added `theme-definitions.mdx` page under /guides to display the themes
  table
- Introduced `ThemesTable` React component for rendering and styling the
  table

Note: Theme definitions are still incomplete and may contain errors.

Author: FlorianNachtigall