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.

tests/test_doc_surfaces.py

@@ -191,16 +191,17 @@ class DownloadLinksKeepTheirAddress(unittest.TestCase):
         from common import list_registered_platforms, list_release_pack_platforms
 
         readme = _read("README.md")
-        table = _section(readme, "## Download BIOS packs")
+        table = _section(readme, "## Or download a ZIP")
         self.assertNotIn("/releases/download/", table)
         released = set(list_release_pack_platforms("platforms"))
+        rows = [line for line in table.splitlines() if line.startswith("| ")]
         for key in list_registered_platforms("platforms"):
             with self.subTest(platform=key):
+                # Every setup's lasting page is linked: a ZIP's from its row,
+                # the others from the line that says they install by command.
                 self.assertIn(f"]({SITE_URL}get/{key}/)", table)
-                row = next(line for line in table.splitlines()
-                           if f"get/{key}/)" in line)
-                if key not in released:
-                    self.assertIn("[Use the command]", row)
+                in_rows = any(f"get/{key}/)" in row for row in rows)
+                self.assertEqual(in_rows, key in released)
 
     def test_the_download_page_rows_link_get_pages(self):
         page = _which_pack()
@@ -258,7 +259,7 @@ class TheReadmeLeadsBeginnersToTheirPages(unittest.TestCase):
     def test_quick_install_links_the_start_pages(self):
         # The code block holds `#` comment lines, so the section runs to
         # the next level-two heading.
-        section = _read("README.md").split("## Quick install", 1)[1].split("\n## ", 1)[0]
+        section = _read("README.md").split("## Install in one line", 1)[1].split("\n## ", 1)[0]
         for page in ("first-install", "troubleshooting", "faq"):
             with self.subTest(page=page):
                 self.assertIn(f"wiki/{page}/)", section)