Important fix for the update mechanism

• Json output ordering fix (using linked-list-map)
• Several smaller bug fixes

• Properly implemented enumerated and nested lists for markdown output.
• Improved error messages for the pulldown_cmark to internal Ast conversion.
• Changed notebook build format to include resource folder (may change again).
• Fixed parsing problem for shortcode arguments with markdown strings.
• Many other small bug fixes.
• Cleanup of unused code and some general refactoring.

This is the first release in a long time. The list of changes is therefore quite long and probably not complete.

Changes:

• The library now uses a custom Ast type for its internal document representation. This greatly simplifies the parsing of shortcodes and math.
• Shortcodes can be nested indefinitely.
• Templates are now specified in .yml files with common metadata. The template sources are either embedded in the .yml file or linked in an external file.
• Templates can be documented through the new syntax. The default template includes an example of how this can be used to produce shortcode documentation.
• Templates now support positional arguments.
• Template arguments are validated. The name and value is checked. Values types are currently either anything or one of a list of values (an enumeration essentially).
• Templates now receive information on shortcodes present in the current document. This makes it possible to create table of content lists with custom shortcodes.
• Rendering and generation pipelines are unified across input and output formats. The notebook input is handled via a separate renderer that wraps the generic renderer internally.
• The system is close to output-format agnostic. Format is now a trait that can be implemented outside the cdoc crate. Perhaps this will be moved to a config file in the future.
• The building process is now parallelized using the rayon library. Additionally, many unnecessary clones have been removed. Rendering now uses the Write trait instead of returning heap strings.

• Improved error handling. This is still only the first step, but the build output is now color-coded and errors are nicely formatted.
• Various smaller fixes. Auto-reload now works properly for both frontmatter and content. Slight improvements to the docs template.

Refactored courses into a document processing crate (cdoc) and a project configuration/management create (courses).