Better installation & nix support

[Quoteme]

we change the installation guide to this:

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Installing

To install the CeTZ package under your local typst package dir you can use the install script from the repository.

Make sure to first perform a recursive git clone

# to fetch a clean clone:
git clone --recursive https://github.com/cetz-package/cetz.git
# or if you forgot the `--recursive` flag before:
git submodule update --init

and also ensure you have just and cargo installed.

For nix users

After enabling flakes, use:

nix --extra-experimental-features 'nix-command flakes' develop

Now use the installation script:

just install

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this solves the issue faced in #637