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.

.github/workflows/ci.yml

@@ -206,9 +206,16 @@ jobs:
           ref: ${{ inputs.version && format('refs/tags/{0}', inputs.version) || github.ref }}
           persist-credentials: false
 
-      - name: Attach the standalone compose file to the release
+      # The installers are served from these assets: docs.ac/install redirects to
+      # releases/latest/download/install.sh (and install.ps1), so the script a
+      # user runs always comes from the newest release.
+      - name: Attach the standalone compose file and the installers to the release
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           TAG: ${{ env.RELEASE_TAG }}
         run: |
-          gh release upload "$TAG" deployment/docker-compose-standalone.yaml --clobber
+          gh release upload "$TAG" \
+            deployment/docker-compose-standalone.yaml \
+            deployment/install.sh \
+            deployment/install.ps1 \
+            --clobber