Rework directives and clean up src directory

[SilasBerger]

Changes

• Implement a unified remark plugin system around remark-container-directives (for container directives) and remark-line-directives (for leaf and text directives).
• Replace all previous remark plugins with the two new ones.
• Move all admonitions and custom MDX to directives syntax, handled by the two new plugins.
• Clean up src dir structure.
• Implement a basic workaround for @site auto-imports within Node code for IntelliJ.

Associated issues

• Closes Move all custom syntax to directives #26
• Closes Colon in Admonitions title causes title to be moved to body #23
• Basic workaround for Add support for @site/... imports in docusaurus.config.ts / builder #2