[FEATURE REQUEST] Folder option for SUMMARY.md

[VaZark]

Is there a standard way to create a Chapter without content or an associated page? Draft chapters assume that the content is not added yet and show a disabled/greyed out text on the sidebar.

What this issue is seeking to address is that while Chapters support a foldable structure but there is no way to use it exclusively as a folder object.

Is it possible to extend the current parser to extract a non-hyperlinked list element as a folder that doesn't have an index and used solely for organisation in the sidebar and the slug? Or conversely, allow Part Title to be a foldable object?

Ex: https://vazark.gitlab.io/til/python/basics/Cheatsheet.html

[joshrotenberg]

Just thinking out loud here, I've only briefly looked at the summary parsing code, but I wonder if this would be possible by making a non-link chapter, so something like this (using the mdbook guide SUMMARY.md as an example):

Currently, the Format section has a top level that is just an index to the subsections:

And the markdown in the SUMMARY.md:

...
- [Format](format/README.md)
    - [SUMMARY.md](format/summary.md)
        - [Draft chapter]()
...

Maybe the parser could support the following format in addition:

- Format
    - [SUMMARY.md](format/summary.md)
        - [Draft chapter]()

Here is the the parsed structure for the Format section currently:

            Link {
                name: "Format",
                location: Some(
                    "format/README.md",
                ),
                number: Some(
                    SectionNumber(
                        [
                            3,
                        ],
                    ),
                ),

So in theory, location: None could indicate this is an unlinked header. 🤷

What this means downstream for renderers is another question, though.

[ehuss]

Thanks for the suggestion! I think this is a duplicate of #483, so closing in favor of that.