Allow 'self' as special entry for root document

[kai-tub]

Context

Hey, I am using this project to manage my furo-based documentation site.
I like it to have a single ToC file that makes it easy to modify the structure and not having to manually edit many sub-tocs.

Like in the sphinx-external-toc documentation I would like to have a separate entry for the landing page; see the discussion over at furo.
The 'Sphinx' way would be to add the special entry self to the toctree call.
From the Sphinx configuration example:

The special entry name self stands for the document containing the toctree directive. This is useful if you want to generate a “sitemap” from the toctree.

From looking at the code of this site, it seems like this particular implementation is solved as an HTML theme option.

Proposal

I am not too familiar with how this library generates the toctree for Sphinx.
Maybe it is possible to reserve self as a special argument to refer to root?
I don't think that this should be an issue for non-root entries, as they are configured via separate entry options.
Only the root document would need to be able to refer to itself to register it as an entry.

Tasks and updates

I also understand that this is a low-priority feature, as the main targets are the executable books project.
But, at least from my viewpoint, it would be a nice feature for this project :)

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