feat: one-command installers for macOS, Linux and Windows

deployment/install.sh (curl | bash) and install.ps1 (irm | iex) check for
Docker, install uv when it is missing or older than 0.8 (pinned 0.12.15 via
Astral's installer), install or upgrade the docsgpt package with
`uv tool install`, and hand the terminal to `docsgpt up` with any arguments.
On Linux without Docker the shell installer offers get.docker.com. Both run
entirely inside a function, so a download cut short runs nothing.

Releases attach both scripts next to the Compose file, which is where
docs.ac/install and docs.ac/install.ps1 will point. installer-lint.yml runs
shellcheck and the PowerShell parser; docker-image-verify.yml now installs
through install.sh. README, Quickstart, Docker-Deploying and the changelog
lead with the one-liner.

docs/content/changelog.mdx

@@ -13,6 +13,13 @@ request, and [Upgrading](/upgrading) covers the steps an existing deployment has
 
 ## Unreleased
 
+### Install with one command
+
+`curl -fsSL https://docs.ac/install | bash` on macOS and Linux, or `irm https://docs.ac/install.ps1 | iex`
+in Windows PowerShell, installs uv and the `docsgpt` package and runs `docsgpt up`. Running it again
+upgrades and keeps your settings. On Linux it offers to install Docker when it is missing. Both
+scripts are attached to every release. See the [Quickstart](/quickstart).
+
 ### `docsgpt up` runs DocsGPT on Docker
 
 The Python package now sets up and runs the Docker stack: `uv tool install docsgpt`, then