Add standalone installer instruction to docs (#12081)

Adopted from `uv` README
(https://github.com/astral-sh/uv#getting-started), this PR adds a
section of using standalone installers in the installation section of
Ruff docs.

docs/installation.md

@@ -13,6 +13,20 @@ ruff check   # Lint all files in the current directory.
 ruff format  # Format all files in the current directory.
 ```
 
+Starting with version `0.5.0`, Ruff can be installed with our standalone installers:
+
+```shell
+# On macOS and Linux.
+curl -LsSf https://astral.sh/ruff/install.sh | sh
+
+# On Windows.
+powershell -c "irm https://astral.sh/ruff/install.ps1 | iex"
+
+# For a specific version.
+curl -LsSf https://astral.sh/ruff/0.5.0/install.sh | sh
+powershell -c "irm https://astral.sh/ruff/0.5.0/install.ps1 | iex"
+```
+
 For **macOS Homebrew** and **Linuxbrew** users, Ruff is also available as [`ruff`](https://formulae.brew.sh/formula/ruff)
 on Homebrew: