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.

CONTRIBUTING.md

@@ -102,7 +102,7 @@ handheld launchers, `bwrap` (bubblewrap) for the probe sandbox, and a Python 3.8
 with pyte (`python3.8 -m pip install "pyte==0.8.2"`) for the full screen. CI has all three.
 
 `profile_sync.py` compares the branch with this repository's `main`, and
-fetches it in a fork's clone. A profile you only edit is not held to
+fetches it in a fork's clone. An edited profile is not held to
 `--strict`: its older refs are reviewed by hand.
 
 The full pipeline is not needed for a contribution: it rebuilds every pack