feat: startingpoint modules (#33)

* feat: add startingpoint modules
some modules authored by @gerblesh
original source: https://github.com/ublue-os/startingpoint/pull/135

* docs: better readme for modules dir

* docs: yafti deps

* docs: sp more information

* feat: startingpoint modules in container

* docs: sentence structure, check sidebar

modules/yafti/README.md

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+# [`yafti`](https://github.com/ublue-os/yafti) Module for Startingpoint
+
+If included, the `yafti` module will install `yafti` and set it up to run on first boot. Also `yafti`'s dependencies, `python3-pip` and `libadwaita` are installed
+
+Optionally, a list of Flatpak names and IDs can be included under `custom-flatpaks:`. These will be enabled by default under their own section on the Flatpak installation screen of `yafti`.
+
+The main `yafti` configuration file, `yafti.yml`, is in `/usr/share/ublue-os/firstboot/yafti.yml` and can be edited for a more custom first-boot experience.
+
+## Example configuration:
+
+```yml
+type: yafti
+custom-flatpaks:
+    - Celluloid: io.github.celluloid_player.Celluloid
+    - Krita: org.kde.krita
+```

modules/yafti/yafti.sh

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+#!/usr/bin/env bash
+
+# Tell build process to exit if there are any errors.
+set -oue pipefail
+
+FIRSTBOOT_DATA="/usr/share/ublue-os/firstboot"
+FIRSTBOOT_LINK="/usr/etc/profile.d/ublue-firstboot.sh"
+
+echo "Installing python3-pip and libadwaita"
+rpm-ostree install python3-pip libadwaita
+
+echo "Installing and enabling yafti"
+pip install --prefix=/usr yafti
+
+# Create symlink to our profile script, which creates the per-user "autorun yafti" links.
+mkdir -p "$(dirname "${FIRSTBOOT_LINK}")"
+ln -s "${FIRSTBOOT_DATA}/launcher/login-profile.sh" "${FIRSTBOOT_LINK}"
+
+YAFTI_FILE="$FIRSTBOOT_DATA/yafti.yml"
+
+get_yaml_array FLATPAKS '.custom-flatpaks[]' "$1"
+if [[ ${#FLATPAKS[@]} -gt 0 ]]; then
+    echo "Adding Flatpaks to yafti.yml"
+    yq -i '.screens.applications.values.groups.Custom.description = "Flatpaks suggested by the image maintainer."' "${YAFTI_FILE}"
+    yq -i '.screens.applications.values.groups.Custom.default = true' "${YAFTI_FILE}"
+
+    for pkg in "${FLATPAKS[@]}"; do
+        echo "Adding to yafti: ${pkg}"
+        yq -i ".screens.applications.values.groups.Custom.packages += [$pkg]" "${YAFTI_FILE}"
+    done
+fi