docs: simplify readme

integrations/doom-emacs/README.md

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+# Doom-Emacs integration
+
+In order to configure Alejandra in
+[Doom-emacs](https://github.com/hlissner/doom-emacs)
+just use the following:
+
+```lisp
+(set-formatter! 'alejandra "alejandra --quiet" :modes '(nix-mode))
+```

integrations/pre-commit-hooks-nix/README.md

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+# Pre-commit-hooks.nix integration
+
+In order to use Alejandra with
+[pre-commit-hooks.nix](https://github.com/cachix/pre-commit-hooks.nix)
+use a configuration file like the following:
+
+```nix
+{
+  pre-commit-check = pre-commit-hooks.lib.${system}.run {
+    hooks = {
+      alejandra.enable = true;
+    };
+  };
+}
+```

integrations/pre-commit/README.md

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+# Pre-Commit integration
+
+In order to use Alejandra with
+[Pre-Commit](https://pre-commit.com/)
+just create a file named `.pre-commit-config.yaml`
+with contents:
+
+```yaml
+repos:
+  - repo: https://github.com/kamadorueda/alejandra
+    rev: 1.4.0
+    # Choose either the 'alejandra' or 'alejandra-system' hook
+    # depending on what pre-requisites you have:
+    hooks:
+      # Requires Nix to be previously installed in the system
+      - id: alejandra
+
+      # Requires Alejandra to be previously installed in the system
+      - id: alejandra-system
+```