Improve SEO: add meta descriptions, robots.txt, and site metadata (#340)

## Summary
- Add missing meta descriptions to the landing page and 5 guide pages
that had none
- Remove redundant "Nebula Docs:" prefix from 3 guide descriptions to
maximize SERP snippet space
- Add `robots.txt` with sitemap reference
- Enrich thin category index descriptions (config, guides, security)
- Update `docusaurus.config.ts`: better tagline, site-wide keywords meta
tag, navbar title for crawlability (visually hidden)
- Add frontmatter blocks to 2 guide pages that had no frontmatter at all

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: bgolat

docs/config/_category_.json

@@ -3,7 +3,7 @@
   "position": 4,
   "link": {
     "type": "generated-index",
-    "description": "Configuration options for Nebula.",
+    "description": "Complete reference for all Nebula configuration options including PKI, firewall rules, lighthouse, relay, tun, and more.",
     "slug": "/config"
   }
 }

docs/guides/_category_.json

@@ -3,7 +3,7 @@
   "position": 2,
   "link": {
     "type": "generated-index",
-    "description": "A collection of how-to guides that explain how to use various capabilities of the Nebula overlay networking tool.",
+    "description": "Step-by-step guides for configuring and troubleshooting Nebula overlay networks, including quick start, certificate management, DNS, and routing.",
     "slug": "/guides"
   }
 }

docs/guides/debug-ssh-commands/index.mdx

@@ -1,5 +1,7 @@
 ---
 title: Debugging with Nebula SSH commands
+description:
+  Learn how to use Nebula's built-in SSH server commands to debug network connectivity issues on overlay hosts.
 summary:
   This guide describes useful commands built into the SSH server accessible over nebula, which can allow debugging
   network connectivity for the nebula host.

docs/guides/quick-start/index.mdx

@@ -1,7 +1,6 @@
 ---
 title: Quick Start
-description:
-  'Nebula Docs: How to create your first overlay network using a Certificate Authority, Lighthouse, and Hosts'
+description: How to create your first overlay network using a Certificate Authority, Lighthouse, and Hosts
 summary:
   This section will walk you through setting up a simple nebula network for testing. The examples will need to be
   modified to suit your particular environment.

docs/guides/rotating-certificate-authority/index.mdx

@@ -1,6 +1,6 @@
 ---
 title: Rotating a Certificate Authority
-description: 'Nebula Docs: How to rotate an expiring certificate authority without downtime.'
+description: How to rotate an expiring Nebula certificate authority without downtime.
 summary:
   This guide will teach you how to migrate from an expiring certificate authority by creating a new certificate
   authority, updating your device's Nebula config to trust the new authority, signing new host certificates, and

docs/guides/sign-certificates-with-public-keys/index.mdx

@@ -1,6 +1,6 @@
 ---
 title: Signing a Certificate Without a Private Key
-description: 'Nebula Docs: How to sign certificates without copying private keys across devices.'
+description: How to sign Nebula certificates without copying private keys across devices.
 summary:
   After reading this guide you will be able to create public/private keypairs on devices you wish to add to the Nebula
   network and generate certificates for them using only the public key.

docs/guides/unsafe_routes/index.mdx

@@ -1,5 +1,8 @@
 ---
 title: Extend network access beyond overlay hosts
+description:
+  Configure Nebula unsafe_routes to route traffic through overlay hosts and reach devices that cannot run Nebula
+  directly.
 summary:
   This guide explains how to configure Nebula to route traffic destined for a specific subnet through a specific overlay
   network host, which is useful for accessing hosts that cannot be modified to run Nebula.

docs/guides/upgrade-to-cert-v2-and-ipv6/index.mdx

@@ -1,5 +1,8 @@
 ---
 title: Upgrading a Nebula network to IPv6 overlay addresses
+description:
+  Step-by-step guide to upgrading an existing Nebula network to the v2 certificate format and enabling IPv6 overlay
+  addresses.
 summary:
   This guide describes how to upgrade an existing nebula network to the v2 certificate format and enable IPv6 addresses.
 ---

docs/guides/using-lighthouse-dns/index.mdx

@@ -1,3 +1,9 @@
+---
+title: Using Lighthouse DNS with Nebula
+description:
+  Configure Nebula's experimental built-in DNS server on Lighthouse hosts to resolve overlay network hostnames.
+---
+
 # Using Experimental Lighthouse DNS with Nebula
 
 :::warning

docs/guides/viewing-nebula-logs/index.mdx

@@ -1,3 +1,8 @@
+---
+title: Viewing Nebula Logs
+description: How to view Nebula logs on Linux, macOS, Windows, iOS, and Android for troubleshooting and monitoring.
+---
+
 import ThemedImage from '@theme/ThemedImage';
 
 # Viewing Nebula Logs