This patch adds support for Hugo's branch and leaf bundles: https://gohugo.io/content-management/page-bundles/
Content directories can now have nested subdirectories, and content from each level will be converted to Gemini. If a subdirectory has a file named index.md (or any index.* file per the footnote on the Hugo page) then it becomes a leaf bundle. For leaf bundles, only the generated index.gmi file will be passed up to parent directories as a page. The rest will be treated as "resources" that are not normally visible to pages in parent folders.
Each parent _index.md receives a list of pages from all nested subdirectories (except for the aforementioned leaf resource pages). This allows for content "rollup" pages. For instance, /series could show all pages under /series/first-season and /series/second-season, while allowing each subfolder to have its own more limited index. This also allows for "clean" URLs, as leaf bundles can be used to make pages that are basically just an index.md page under a named folder. You can then use the trimSuffix function (from sprig) in your gotmpl templates to clean up any links, by stripping "/index.md".
This patch also adds the copying of non-Markdown resource files from content directories, so that bundled resources can be pulled in. Theoretically if someone puts carefully named .gmi files in their content folder then this could cause a name collision, but you'd almost have to try to make that happen, so I didn't put in any checks for it. If this happens then any generated files with the same name are simply overwritten, so there's no serious harm done.
Hugo's leaf bundles support a special front matter attribute, "headless", which if set to true will cause the index.md file to be "invisible" and it will not be passed up to the parent. This is not currently implemented.
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