docs: Auto generate command list

docs/CONTRIBUTING.md

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+# Contributing
+
+Contributors are very welcome! **No contribution is too small and all contributions are valued.**
+
+Some suggestions to get started:
+
+- You can look at the [good first issue][good-first-issue] label on the issue tracker.
+- Help with packaging on various distributions needed!
+- To use print debugging to the [Helix log file][log-file], you must:
+  * Print using `log::info!`, `warn!`, or `error!`. (`log::info!("helix!")`)
+  * Pass the appropriate verbosity level option for the desired log level. (`hx -v <file>` for info, more `v`s for higher severity inclusive)
+- If your preferred language is missing, integrating a tree-sitter grammar for
+    it and defining syntax highlight queries for it is straight forward and
+    doesn't require much knowledge of the internals.
+
+We provide an [architecture.md][architecture.md] that should give you
+a good overview of the internals.
+
+# Auto generated documentation
+
+Some parts of [the book][docs] are autogenerated from the code itself,
+like the list of `:commands` and supported languages. To generate these
+files, run
+
+```shell
+cargo xtask bookgen
+```
+
+inside the project. We use [xtask][xtask] as an ad-hoc task runner and
+thus do not require any dependencies other than `cargo` (You don't have
+to `cargo install` anything either).
+
+[good-first-issue]: https://github.com/helix-editor/helix/labels/E-easy
+[log-file]: https://github.com/helix-editor/helix/wiki/FAQ#access-the-log-file
+[architecture.md]: ./architecture.md
+[docs]: https://docs.helix-editor.com/
+[xtask]: https://github.com/matklad/cargo-xtask