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/hosts.ejs

@@ -419,7 +419,7 @@
 				<span class="card-icon me-2"><i class="fa-solid fa-network-wired"></i></span>
 				<span class="card-title fw-bold">Proxy List</span>
 				<span class="ms-auto">
-					<a href="/docs/installation" class="text-reset me-2" title="Help"><i class="fa-solid fa-circle-question"></i></a>
+					<a href="/docs/hosts" class="text-reset me-2" title="Help"><i class="fa-solid fa-circle-question"></i></a>
 					<button type="button" class="btn btn-sm btn-outline-secondary me-2" onclick="hostClearCache(this)" title="Clear cached wildcard subdomain lookups">
 						<i class="fa-solid fa-broom"></i>
 						Clear cache
@@ -553,7 +553,7 @@
 		<div class="modal-content card border-0">
 			<div class="modal-header">
 				<h5 class="modal-title" id="hostModalTitle">Add host</h5>
-				<a href="/docs/installation" class="text-reset me-2" title="Help"><i class="fa-solid fa-circle-question"></i></a>
+				<a href="/docs/hosts" class="text-reset me-2" title="Help"><i class="fa-solid fa-circle-question"></i></a>
 				<button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
 			</div>