docs: archive recipes page and README badges

Every archive has a recipe in recipes/archives.json, and no page a user
reads said so. A short reference page says what a recipe is, how to
rebuild one archive and what exact and canonical mean. Its counts and the
README line are counted from the recipe file at each build.

The README gains a row of four badges: PyPI version, Python versions,
the Validation workflow on main and the license. Tests hold the badges,
the counts and the page's command.

.github/workflows/validate.yml

@@ -32,6 +32,7 @@ on:
       - "release.json"
       - "derivable.json"
       - "retired.json"
+      - "recipes/**"
       - "measurements/**"
       - "website/**"
       # The type check's configuration and baseline.
@@ -62,6 +63,7 @@ on:
       - "release.json"
       - "derivable.json"
       - "retired.json"
+      - "recipes/**"
       - "measurements/**"
       - "website/**"
       - "pyproject.toml"