feat: sitemap and robots

robots.txt

@@ -0,0 +1,3 @@
+User-agent: *
+Allow: /
+Sitemap: https://blog.zxcvber.com/sitemap.xml

sitemap.xml

@@ -0,0 +1,28 @@
+---
+layout: null
+---
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  {% for post in site.posts %}
+    <url>
+      <loc>{{ site.url }}{{ post.url }}</loc>
+      {% if post.last_modified_at == null %}
+        <lastmod>{{ post.date | date_to_xmlschema }}</lastmod>
+      {% else %}
+        <lastmod>{{ post.last_modified_at | date_to_xmlschema }}</lastmod>
+      {% endif %}
+
+      {% if post.sitemap.changefreq == null %}
+        <changefreq>weekly</changefreq>
+      {% else %}
+        <changefreq>{{ post.sitemap.changefreq }}</changefreq>
+      {% endif %}
+
+      {% if post.sitemap.priority == null %}
+        <priority>0.5</priority>
+      {% else %}
+        <priority>{{ post.sitemap.priority }}</priority>
+      {% endif %}
+    </url>
+  {% endfor %}
+</urlset>