Add plain-language concept docs; fix docs viewer rendering; link API tokens

- New docs/concepts-{hosts,dns,access,api-tokens}.md -- plain-language
  guides aimed at less technical readers, each linking onward to the
  existing system-design-level doc for anyone who wants that detail.
  Card help links (Proxy List, Add/Edit host, DNS Provider cards,
  Users/Permissions/Groups cards) now point here instead of straight at
  Installation/Architecture.
- The "New API Token" card had no help link at all -- added, pointing to
  the new API Tokens doc.
- Fixed the in-app docs viewer rendering every docs/*.md page with a
  garbled heading + stray <hr> at the top: Jekyll front matter (meant
  only for the GitHub Pages build) was never stripped before being
  handed to the markdown renderer.
- Fixed cross-doc links never resolving in-app, since this viewer serves
  docs at /docs/<slug> with no .html suffix: rewritten to the correct
  in-app URL, first by registered slug, falling back to the doc's real
  filename (the correct, working link form on the Jekyll/GitHub Pages
  build) -- same idea as the existing image-path fix, and lets one link
  written in a doc work on both targets.

Bumps to v1.1.13.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KDEx8ghuZR61pqPXc6da9C

nodejs/views/groups.ejs

@@ -86,7 +86,7 @@
 			<div class="card-header text-center">
 				<span class="card-icon float-start"><i class="fa-solid fa-users-gear"></i></span>
 				<span class="card-title">Add Group</span>
-				<a href="/docs/architecture" class="text-reset float-end" title="Help"><i class="fa-solid fa-circle-question"></i></a>
+				<a href="/docs/access" class="text-reset float-end" title="Help"><i class="fa-solid fa-circle-question"></i></a>
 			</div>
 			<div class="card-header actionMessage" style="display:none"></div>
 			<div class="card-body">