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

docs/architecture.md

@@ -8,6 +8,11 @@ description: How the proxy's OIDC client, LDAP client, and OpenResty routing fit
 
 [← Back to Home](index.html)
 
+> Looking for a plainer explanation of hosts, HTTPS, or the local
+> permission model instead of internals? See
+> [Hosts & HTTPS](concepts-hosts.html) and
+> [Users, Groups & Permissions](concepts-access.html).
+
 ## System Overview
 
 The proxy system consists of three main components working together to provide high-performance reverse proxying with automated SSL management.