fix: let the site search land on cards

The search splits a page at its headings. Emulator cards were titled in
bold and platform systems folded into blocks under one heading, so a
pasted hash opened the top of a page or the systems overview of a
100-block platform page. Each file card is now a level 4 heading kept
out of the table of contents, and each platform system a heading.

Card lines that list other pages (platforms, packs, source citations,
collection paths) leave the search index: 24.7 MB down to 18.1 MB, the
names, hashes and notes still searchable.

tests/test_site_system_links.py

@@ -143,7 +143,7 @@ class AnIdNamesOneCardPerPage(unittest.TestCase):
                  "path": "bios/Nintendo/DS/a.bin"}
         page = gs.generate_system_page(
             "Nintendo", {"DS": [entry], "DSi": [entry]}, FileConsumers({"files": {}}))
-        self.assertEqual(page.count('id="sha1-aaaaaaaaaaaa"'), 1)
+        self.assertEqual(page.count("#sha1-aaaaaaaaaaaa .rb-file-title"), 1)
         self.assertEqual(page.count("rb-sys-file"), 2)