feat(seo): add llms.txt, robots.txt, Twitter cards, and Open Graph tags

- Add llms.txt (2.5KB summary for LLM crawlers)
- Add llms-full.txt (182KB full docs content for LLM context)
- Add robots.txt with sitemap reference
- Add Twitter card meta tags to all docs pages
- Update theme-color to brand purple
- Hugo already generates sitemap.xml and Open Graph tags

Author: dndungu

hugo.yaml

@@ -4,13 +4,18 @@ title: Zerfoo - ML Framework for Go
 theme: hugo-book
 
 params:
+  description: "Production-grade ML inference and training framework for Go. 245 tok/s on Gemma 3 1B. Zero CGo. 20 model architectures."
   BookTheme: auto
   BookToC: true
   BookSection: docs
   BookLogo: /logo.png
   BookMenuBundle: /menu
   BookSearch: true
   BookDateFormat: January 2, 2006
+  images:
+    - /zerfoo.png
+
+enableRobotsTXT: true
 
 markup:
   goldmark:
@@ -19,3 +24,11 @@ markup:
   highlight:
     style: monokai
     lineNos: false
+
+outputs:
+  home:
+    - HTML
+    - RSS
+  section:
+    - HTML
+    - RSS

layouts/_partials/docs/html-head.html

@@ -6,10 +6,18 @@
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <meta name="description" content="{{ or .Description .Summary | plainify | htmlUnescape }}">
-<meta name="theme-color" media="(prefers-color-scheme: light)" content="#ffffff">
-<meta name="theme-color" media="(prefers-color-scheme: dark)" content="#2e3440">
+<meta name="theme-color" media="(prefers-color-scheme: light)" content="#7C3AED">
+<meta name="theme-color" media="(prefers-color-scheme: dark)" content="#8B5CF6">
 <meta name="color-scheme" content="light dark">
 
+<!-- Twitter Card -->
+<meta name="twitter:card" content="summary_large_image">
+<meta name="twitter:title" content="{{ partial "docs/html-head-title" . }}">
+<meta name="twitter:description" content="{{ or .Description .Summary | plainify | htmlUnescape | truncate 200 }}">
+{{- with .Site.Params.images }}
+<meta name="twitter:image" content="{{ index . 0 | absURL }}">
+{{- end }}
+
 {{- with .Page.Params.BookHref -}}
   <meta http-equiv="Refresh" content="0; url='{{ . }}'" />
 {{- end -}}