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.

platforms/_guides.yml

@@ -19,7 +19,8 @@
 # The registry keeps what tools read (device_group, extract_to, install);
 # this file keeps what people read.
 
-install_base: https://raw.githubusercontent.com/Abdess/retrobios/main
+# The site serving the one-line bootstraps (scripts/sitebootstrap.py).
+install_base: https://abdess.github.io/retrobios
 
 # Is it safe: shown on the home page and under every command on a guide page.
 # Where it writes follows install.py: the BIOS folder, the frontend's save