Suggestion: Add Tree View for File Navigation

Hi 👋,

First of all, thanks for this amazing project! I really enjoyed how `rendergit` flattens a GitHub repository into a single HTML file with all the contents accessible.

Currently, the left-hand Contents sidebar lists all files in a single flat structure. While this provides quick access, it becomes difficult to navigate in larger repositories because the directory hierarchy is lost, making it harder to understand the project’s organization at a glance.

**Suggestion 1 :** It would be very useful if the contents sidebar could display files in a **tree format**, mirroring the folder structure.
This would make navigation easier to relate to and more intuitive.

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**Suggestion 2 :** Each file in the directory tree could also be a hyperlink directly to its section in the rendered HTML.

The issue does not name files, tests, or an entry point. Start by locating the code that builds the left-hand Contents sidebar, then define the directory tree and file anchors; done means the sidebar mirrors the repository hierarchy and each file links to its rendered section.

Written by the indexing model from the issue text.