Broken links in GitBook SUMMARY.md when chunk="to-content"

[infotexture]

When generating GitBook output with --format=markdown_gitbook, the ToC contains broken links for topics that are chunked to content, like in the DITA-OT docs glossary submap.

See https://infotexture.github.io/ot-mkdocs-demo/reference/glossary/ for sample output.

For the corresponding summary file output, see https://github.com/infotexture/ot-mkdocs-demo/blob/master/docs/SUMMARY.md?plain=1#L209-L221.

The problem seems to be that the markdown_gitbook output creates extensionless filenames on disk with the ID attribute as part of the filename, like glossary.md#gloss-argument and others.

[infotexture]

See also #167.