Add blog via Astro content collections

Introduce blog support: content collection schema, listing and post
routes, and a sample Markdown post. Update docs and TODO; add blog
assets dir and adjust color variables in docs. Also set
absolute_redirect off in nginx.conf for container routing.

src/pages/index.astro

@@ -111,6 +111,16 @@ if (now < new Date(now.getFullYear(), 6, 8)) age--;
                 </div>
             </section>
 
+            <div class="divider">══════════════════════════════</div>
+
+            <!-- Blog -->
+            <section class="section fade-in">
+                <h2>Blog</h2>
+                <ul class="links">
+                    <li><a href="/blog">blog</a> — thoughts and things</li>
+                </ul>
+            </section>
+
             <!-- Footer -->
             <footer class="footer fade-in">
                 <div class="divider">══════════════════════════════</div>