docs: Add sitemap.xml, robots.txt.

Also, while we're here, cleanup the favicon and apple-touch-icon
duplication mess.

docs/_config.yml

@@ -1,6 +1,16 @@
 include: [.well-known]
 exclude: [bin, vendor, CNAME, Gemfile, Gemfile.lock]
 
+# Same as GitHub Pages
+# https://help.github.com/articles/using-jekyll-with-pages#troubleshooting
+# Disable despite enabled on GitHub Pages for supported plugins to work.
+# safe: true
+kramdown:
+  input: GFM
+  hard_wrap: false
+lsi: false
+highlighter: rouge
+
 gems:
-  - jekyll-feed
+  - jekyll-sitemap
   - jekyll-seo-tag

docs/_layouts/base.html

@@ -12,19 +12,8 @@
     {% seo title=false %}
     {% feed_meta %}
     <meta name="viewport" content="width=device-width">
-    <link rel="shortcut icon" href="/img/favicon.ico">
-    <link rel="icon" sizes="16x16" href="/img/favicon-16x16.png" >
-    <link rel="icon" sizes="32x32" href="/img/favicon-32x32.png">
-    <link rel="icon" sizes="96x96" href="/img/favicon-96x96.png">
-    <link rel="icon" sizes="192x192" href="/img/favicon-192x192.png">
-    <link rel="apple-touch-icon-precomposed" sizes="57x57" href="/img/apple-touch-icon-57x57-precomposed.png">
-    <link rel="apple-touch-icon-precomposed" sizes="72x72" href="/img/apple-touch-icon-72x72-precomposed.png">
-    <link rel="apple-touch-icon-precomposed" sizes="76x76" href="/img/apple-touch-icon-76x76-precomposed.png">
-    <link rel="apple-touch-icon-precomposed" sizes="114x114" href="/img/apple-touch-icon-114x114-precomposed.png">
-    <link rel="apple-touch-icon-precomposed" sizes="120x120" href="/img/apple-touch-icon-120x120-precomposed.png">
-    <link rel="apple-touch-icon-precomposed" sizes="144x144" href="/img/apple-touch-icon-144x144-precomposed.png">
-    <link rel="apple-touch-icon-precomposed" sizes="152x152" href="/img/apple-touch-icon-152x152-precomposed.png">
-    <link rel="apple-touch-icon-precomposed" sizes="180x180" href="/img/apple-touch-icon-180x180-precomposed.png">
+    <link rel="icon" type="image/x-icon" href="/img/favicon.ico">
+    <link rel="apple-touch-icon"href="/img/apple-touch-icon.png">
     <link rel="stylesheet" href="/css/screen.css" type="text/css" media="screen">
     <link rel="stylesheet" href="/css/pygments.css" type="text/css" media="screen">
     <script>

docs/robots.txt

@@ -0,0 +1,4 @@
+---
+---
+User-agent: *
+Sitemap: {{ site.url }}/sitemap.xml