Add developer docs [skip ci]

docs/maintenance.md

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+# ToriLinux maintenance (developer documentation)
+
+## Working with projects
+
+### Update project versions
+
+You can use `.ci/update.py` to update project versions.
+
+### Add/modify/delete a project
+
+You can add/remove/modify the script(s) in `airootfs/root/customize_airootfs/scripts`.
+
+For now, you can use other scripts for reference.
+
+Use `.ci/configure.py [cuda|rocm] [gui|normal]` to render these templates.
+
+## Working with CI
+
+Modify any of the templates in `.ci/templates/` or `.ci/actions.py`.
+
+Use `.ci/actions.py` to regenerate CI workflows in `.github/actions/`.
+
+## Build locally
+
+You need ArchLinux or at least any linux distribution that supports Docker (you can use a Docker image of ArchLinux).
+
+Install the dependencies listed in the `.ci/dependencies.sh` file.
+
+Render the scripts using `.ci/configure.py [cuda|rocm] [gui|normal]`.
+
+Optionally, you can run `.ci/empty.sh` after `.ci/configure.py` to create an empty image.
+
+Run `mkarchiso -v -m iso -w /some/temporary/directory -o /some/output/directory .` to build the image.
+
+Preferably use tmpfs (e.g., `/tmp/mkarchiso`) for the temporary directory (note that you need more than 50 GB of free space in that directory).