Add custom markdown pages feature to web dashboard

Allows adding static content pages (About, FAQ, etc.) as markdown files
with YAML frontmatter. Pages are stored in PAGES_HOME directory (default:
./pages), automatically appear in navigation menu, and are included in
the sitemap.

- Add PageLoader class to parse markdown with frontmatter
- Add /pages/{slug} route for rendering custom pages
- Add PAGES_HOME config setting to WebSettings
- Add prose CSS styles for markdown content
- Add pages to navigation and sitemap
- Update docker-compose.yml with pages volume mount
- Add comprehensive tests for PageLoader and routes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

example/pages/about.md

@@ -0,0 +1,29 @@
+---
+title: About
+slug: about
+menu_order: 10
+---
+
+# About Our Network
+
+Welcome to our MeshCore mesh network! This page demonstrates the custom pages feature.
+
+## What is MeshCore?
+
+MeshCore is an open-source off-grid LoRa mesh networking platform. It enables peer-to-peer communication without relying on traditional internet or cellular infrastructure.
+
+## Our Mission
+
+Our community-operated network aims to:
+
+- Provide resilient communication during emergencies
+- Enable outdoor enthusiasts to stay connected in remote areas
+- Build a community of mesh networking enthusiasts
+
+## Getting Started
+
+To join our network, you'll need:
+
+1. A compatible LoRa device (T-Beam, Heltec, RAK, etc.)
+2. MeshCore firmware installed
+3. The correct radio configuration for our region