Docs for PR 9373

Author: jkrumbiegel

docs/computations/julia.qmd

@@ -210,6 +210,10 @@ To enable the cache for a document, add the `cache` option. For example:
 
 The `julia` engine uses the [QuartoNotebookRunner.jl](https://github.com/PumasAI/QuartoNotebookRunner.jl/) package to render notebooks. When you first attempt to render a notebook with the `julia` engine, Quarto will automatically install this package into a private environment that is owned by Quarto. This installation will therefore not interact or conflict with any other Julia environments on your machine. Quarto will use the `julia` binary on your PATH by default, but you can override this using the `QUARTO_JULIA` environment variable.
 
+#### Using custom versions of `QuartoNotebookRunner`
+
+In special circumstances, you may not want to use the specific `QuartoNotebookRunner` version that quarto installs for you. One reason could be that you're developing `QuartoNotebookRunner` itself, or you need to use a fork or an unreleased version with a bugfix. In this case, you can run quarto with the environment variable `QUARTO_JULIA_PROJECT` set to a directory of a julia environment that has `QuartoNotebookRunner` installed. As an example, you could install the main branch of `QuartoNotebookRunner` into the directory `/some/dir` by executing `]activate /some/dir` in a julia REPL and then `]add QuartoNotebookRunner#main`. If there is no server process running, yet, after running a command like `QUARTO_JULIA_PROJECT=/some/dir quarto render some_notebook.qmd` in your terminal, the server process should be started using the custom `QuartoNotebookRunner`. You can also set quarto's `--execute-debug` flag and check the output to verify that the custom environment is being used.
+
 ### Rendering notebooks
 
 To use the `julia` engine, you have to specify it in your frontmatter: