docs: sentence-case and split README paragraphs

Quick Install was the one Title Case heading left in the published
text. The Files sentence was glued to the paragraph above it, two
paragraphs before the table whose column it describes. It now names
the column and sits just above the table.

tests/test_doc_surfaces.py

@@ -185,6 +185,36 @@ class ReleaseCommitCarriesTheManifests(unittest.TestCase):
         self.assertIn("install/*.json", add.split())
 
 
+class PublishedHeadings(unittest.TestCase):
+    """README.md kept '## Quick Install', the one Title Case heading left,
+    because the casing test read wiki/ only."""
+
+    def test_readme_and_contributing_use_sentence_case(self):
+        offenders = []
+        for page in (ROOT / "README.md", ROOT / "CONTRIBUTING.md"):
+            text = FENCED.sub("", page.read_text(encoding="utf-8"))
+            for title in re.findall(r"(?m)^#{1,4} (.+)$", text):
+                if _sentence_case(title) != title:
+                    offenders.append(f"{page.name}: {title}")
+        self.assertEqual(offenders, [], "\n".join(offenders))
+
+
+class FilesColumnParagraph(unittest.TestCase):
+    """'Files is how many the pack holds' was glued to the paragraph above
+    it, two paragraphs before the table whose column it described."""
+
+    def test_it_names_the_column_in_its_own_paragraph_before_the_table(self):
+        import generate_readme
+
+        source = inspect.getsource(generate_readme)
+        self.assertNotIn("Files is how many", source)
+        readme = (ROOT / "README.md").read_text(encoding="utf-8")
+        paragraph = next(p for p in readme.split("\n\n") if "The Files column" in p)
+        self.assertNotIn("\n", paragraph.strip())
+        after = readme.split(paragraph, 1)[1].lstrip("\n")
+        self.assertTrue(after.startswith("| Platform | Files |"))
+
+
 class PageOpenings(unittest.TestCase):
     """Four pages opened by announcing what they cover."""