Document Homebrew and Scoop, and fix the deb/rpm filenames

The README never mentioned the package managers and the docs covered Homebrew
but not Scoop, even though both ship from every release. Add the Homebrew
(macOS) and Scoop (Windows) channels to both. Fix the Linux package globs:
GoReleaser names them kage_<ver>_amd64.deb and kage-<ver>-1.x86_64.rpm, not
kage_*_linux_amd64.{deb,rpm}, so the documented commands matched nothing.

docs/content/getting-started/installation.md

@@ -1,6 +1,6 @@
 ---
 title: "Installation"
-description: "Install kage from Go, Homebrew, a release archive, a Linux package, or the container image, and point it at a browser."
+description: "Install kage from Go, Homebrew, Scoop, a release archive, a Linux package, or the container image, and point it at a browser."
 weight: 20
 ---
 
@@ -12,12 +12,22 @@ kage is a single binary. Pick whichever channel suits you.
 go install github.com/tamnd/kage/cmd/kage@latest
 ```
 
-## Homebrew
+## Homebrew (macOS)
 
 ```bash
 brew install tamnd/tap/kage
 ```
 
+The cask installs the prebuilt macOS binary. On Linux, use the packages below or
+`go install`.
+
+## Scoop (Windows)
+
+```bash
+scoop bucket add tamnd https://github.com/tamnd/scoop-bucket
+scoop install kage
+```
+
 ## Release archives and Linux packages
 
 Every [release](https://github.com/tamnd/kage/releases) attaches `tar.gz`
@@ -28,10 +38,10 @@ signature. Download the one for your platform, extract `kage`, and put it on you
 
 ```bash
 # Debian/Ubuntu
-sudo dpkg -i kage_*_linux_amd64.deb
+sudo dpkg -i kage_*_amd64.deb
 
 # Fedora/RHEL
-sudo rpm -i kage_*_linux_amd64.rpm
+sudo rpm -i kage-*.x86_64.rpm
 ```
 
 ## Container