docs: turn the readme into a front page

The README ran 211 lines and 2,000 words: five commands with options,
an 18-row download table, coverage figures and the build commands, all
before the help. It now says what the project does, gives the line that
installs for Windows and for the rest, shows it, lists the six ZIPs and
the setups that install by command, and leads to the site, where the
figures and every option live. The build commands sit in a closed block.
No personal pronoun is left in its text.

scripts/site_markdown.py

@@ -20,11 +20,15 @@ on the built site:
 from __future__ import annotations
 
 import argparse
+import os
 import re
 import sys
 from html.parser import HTMLParser
 from pathlib import Path
 
+sys.path.insert(0, os.path.dirname(__file__))
+from common import run_cli  # noqa: E402
+
 SITE = "https://abdess.github.io"
 BASE = "/retrobios/"
 # Elements whose content is never words a reader needs.
@@ -342,4 +346,4 @@ def main() -> int:
 
 
 if __name__ == "__main__":
-    sys.exit(main())
+    run_cli(main)