Website: fix broken /install.html link, attempt #3

Commit 3af523e fixed redirects for
/cli.html, /hiring.html, and /license.html, but missed /install.html.
Add a redirect page for /install.html.

Author: strager

website/install.md

@@ -20,6 +20,7 @@ cp gnome-terminal.svg "${output_directory}/"
 cp hiring.html "${output_directory}/"
 cp homebrew.svg "${output_directory}/"
 cp index.html "${output_directory}/"
+cp install.html "${output_directory}/"
 cp license.html "${output_directory}/"
 cp linux.svg "${output_directory}/"
 cp macos.svg "${output_directory}/"