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.

mkdocs.yml

@@ -82,6 +82,9 @@ markdown_extensions:
 - tables
 - toc:
     permalink: true
+    # File cards are level 4 headings: the search lands on them, and a
+    # page of a hundred cards keeps a readable table of contents.
+    toc_depth: 3
 - pymdownx.details
 - pymdownx.highlight:
     anchor_linenums: true