feat: serve the one-line installer from the site

The site serves install.sh at /retrobios/i and install.ps1 at
/retrobios/i.ps1. generate_site.py writes both copies with the
installer address set to install.py at the commit the site is built
from (GITHUB_SHA in CI, HEAD locally), so the pinned SHA-256 and the
downloaded installer always come from one commit, including while a
deploy is still running.

Every command a user copies names the short address. The raw form
stays on the installer page as the fallback. Deploy Site now runs on
changes to install.sh and install.ps1. The install box weight keeps
the address length its ratio was fitted on, so no console page moves.

scripts/generate_readme.py

@@ -35,6 +35,7 @@ from common import (
     write_if_changed,
 )
 import release_record
+from siteguides import install_command
 from verify import verify_platform
 
 
@@ -557,21 +558,21 @@ def generate_readme(db: dict, platforms_dir: str, db_path: str = "database.json"
         "",
         "```bash",
         "# Linux / macOS / Steam Deck",
-        "curl -fsSL https://raw.githubusercontent.com/Abdess/retrobios/main/install.sh | sh",
+        install_command("sh"),
         "",
         "# Windows (PowerShell)",
-        "irm https://raw.githubusercontent.com/Abdess/retrobios/main/install.ps1 | iex",
+        install_command("powershell"),
         "",
         "# Android (Termux, after pkg install python and termux-setup-storage)",
-        "curl -fsSL https://raw.githubusercontent.com/Abdess/retrobios/main/install.sh | sh",
+        install_command("sh"),
         "",
         "# SD card mounted on a PC: a muOS, KNULLI, ROCKNIX, Onion, spruceOS or MinUI card",
         "# is found by itself. For another card, --dest is its BIOS folder",
         "# (RetroArch on Switch: retroarch/cores/system, on Android: RetroArch/system)",
-        "curl -fsSL https://raw.githubusercontent.com/Abdess/retrobios/main/install.sh | sh -s -- --platform retroarch --dest /path/to/sdcard/retroarch/cores/system",
+        install_command("sh", "--platform retroarch --dest /path/to/sdcard/retroarch/cores/system"),
         "",
         "# Options in PowerShell: anything after | iex goes to iex, so the script runs as a block",
-        "&([scriptblock]::Create((irm https://raw.githubusercontent.com/Abdess/retrobios/main/install.ps1))) --check",
+        install_command("powershell", "--check"),
         "```",
         "",
         "The script opens a full-screen installer, driven with the arrow keys and Enter, or with a"