Fix three ways the index misread its own data

An advisory copied from a GitHub Security Advisory opens with "### Impact"
or "### Summary". The section parser treated any heading as a boundary, so
those descriptions ended themselves and reached the site empty. Only the
four headings an entry actually uses end a section now, and leading heading
markers are stripped when the text is rendered.

A trailing slash made exploit-db.com/exploits/41034 and .../41034/ two
different links, which put 53 duplicate rows in front of readers.

github.com also serves attachments and advisories, and 103 of those were
being parsed as owner/repo -- rendering "user-attachments / files" with an
empty star column instead of the PoC archive they point to.

docs/logic.js

@@ -72,9 +72,14 @@ function shortAge(hours) {
   return Math.round(days / 365) + 'y';
 }
 
+// github.com serves more than repositories; an attachment or an advisory has
+// no owner and no stars, and reads as nonsense in an owner / repo row.
+const NOT_A_REPO = new Set(['user-attachments', 'advisories', 'security', 'orgs',
+  'apps', 'marketplace', 'sponsors', 'topics', 'collections', 'settings', 'notifications']);
+
 function repoFromUrl(url) {
   const match = /^https?:\/\/(?:www\.)?github\.com\/([^/#?]+)\/([^/#?]+)/i.exec(url || '');
-  if (!match) return null;
+  if (!match || NOT_A_REPO.has(match[1].toLowerCase())) return null;
   return { owner: match[1], repo: match[2].replace(/\.git$/i, '') };
 }