docs: 📝 add installation guide

docs/guide/installation.qmd

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+---
+title: "Installing"
+order: 0
+---
+
+Before installing `check-datapackage`, you need to have
+[Python](https://www.python.org/downloads/) and
+[pipx](https://pipx.pypa.io/latest/installation/) installed.
+
+To check that these programs are installed, run the following commands
+in your Terminal:
+
+``` {.bash filename="Terminal"}
+python3 --version
+pipx --version
+```
+
+If Python and pipx are installed, these commands will show you the
+versions installed on your system. If you get an error, you need to
+install them before continuing.
+
+## Install in a project (recommended)
+
+It's generally recommended to install Python packages in a project's
+virtual environment to keep the project's dependencies isolated from the
+system-wide Python setup and avoid conflicts. There are several tools to
+manage package dependencies and create virtual environments, such as
+venv, virtualenv, and uv. For this guide, we will use
+[uv](https://docs.astral.sh/uv/).
+
+If you don't have uv installed, you can install it in your Terminal with
+pipx:
+
+``` {.bash filename="Terminal"}
+pipx install uv
+```
+
+To check that uv is installed, run this command:
+
+``` {.bash filename="Terminal"}
+uv
+```
+
+If you get a list of uv commands, you've installed it successfully. With
+uv installed, you can now install `check-datapackage` directly from the
+[GitHub
+repository](https://github.com/seedcase-project/check-datapackage):
+
+``` {.bash filename="Terminal"}
+uv add check-datapackage
+```
+
+You can check that `check-datapackage` has been installed correctly by
+running this command:
+
+``` {.bash filename="Terminal"}
+uv pip show check_datapackage
+```
+
+If `check-datapackage` has been installed successfully, the output will
+show details about `check-datapackage`.
+
+## Install system-wide
+
+We strongly recommend using `check-datapackage` in a virtual
+environment, but you can also install it system-wide. The easiest way to
+do that is to use pipx:
+
+``` {.bash filename="Terminal"}
+pipx install check-datapackage
+```
+
+To check that `check-datapackage` has been installed correctly, run the
+following and make sure `check_datapackage` is listed:
+
+``` {.bash filename="Terminal"}
+pipx list
+```
+
+Now you can use `check-datapackage` in any Python script on your
+computer.