Add support for custom content trees similarly to Docusaurus Sidebars

[alfredbaudisch]

When it's needed to fine-tune a content tree, it's better to manually create them. Add support to the property custom_content_trees into the root _index.md.

custom_content_trees is a map, where each key is a content key identifier:

%{ custom_content_trees: %{
  "custom-tree": []
}}

Then the custom tree should be available by calling Repository.get_content_tree("custom-tree").

The custom tree should have support to:

• Custom external and internal links
• Slug and taxonomy links, where when the tree is built, a slug Title should be automatically fetch
  • It should be possible to override the title in the custom tree
• Custom nesting and custom group/folder name

Example: https://docusaurus.io/docs/sidebar

[alfredbaudisch]

This is not related to PardallMarkdown itself, it should be implemented by a consuming application itself.