docs: add and enhance documentation for gui, setup and usage (#34)

docs/index.md

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+# Universal Blue - Forge Documentation
+
+On-premises Universal Blue provides the service units necessary to set up a self-hosted
+OS forge for custom images.
+
+## Configuration
+
+Since we are using ansible in the background, all user configurations are loaded via
+[`extra-vars`](https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_variables.html#defining-variables-at-runtime)
+at runtime. No worries on how things are plugged together, you only have to know where
+to create your configuration files. That's all.
+
+So, during forge setup a podman volume `ublue-os_forge-data` is created. All user configuration
+etc. is stored there. You can inspect the volume with:
+
+```sh
+❯ podman volume inspect ublue-os_forge-data
+[
+     {
+          "Name": "ublue-os_forge-data",
+          "Driver": "local",
+          "Mountpoint": "/var/home/stephan/.local/share/containers/storage/volumes/ublue-os_forge-data/_data",
+          "CreatedAt": "2024-05-06T21:25:28.818751853+02:00",
+          "Labels": {},
+          "Scope": "local",
+          "Options": {},
+          "MountCount": 0,
+          "NeedsCopyUp": true,
+          "LockNumber": 98
+     }
+]
+```
+
+Under the `"Mountpoint:"` you find the actual path on your file system. I would recommend you
+symlink this folder to a path more rememberable. For example you can do the following:
+
+```sh
+FORGE_POD_DATA_DIR="$(podman volume inspect ublue-os_forge-data | jq -r '.[0].Mountpoint')"
+ln -s $FORGE_POD_DATA_DIR $HOME/ublue-os_forge-data
+```
+
+With this the data folder would be available in your home directory under `~/ublue-os_forge-data`
+
+In that folder you will find an example configuration similar to this:
+
+```yaml
+## ublue-os forge extra-vars example configuration
+## For more details got to https://github.com/ublue-os/forge/blob/main/docs/variables.md
+---
+forge_git_repository_url: https://github.com/ublue-os/bluefin.git
+forge_git_repository_destination: /var/home/stephan/.local/share/containers/storage/volumes/ublue-os_forge-data/_data/data/bluefin
+forge_git_repository_version: main
+forge_registry_url: registry.ublue.local
+```
+
+This file acts as a template. It has all available settings outlined for you. Simple copy
+it and modify it to your liking. For each project you want to handle with the forge you can
+create a dedicated file.
+
+Details about the available variables are documented [here](./variables.md).
+
+## Usage
+
+There are two methods of operating the forge, either via a [GUI](./gui.md) available
+at <https://forge.ublue.local> or via [just](./just.md) command runner.