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/content/config.ts

@@ -0,0 +1,13 @@
+import { z, defineCollection } from 'astro:content';
+
+const blog = defineCollection({
+  type: 'content',
+  schema: z.object({
+    title: z.string(),
+    date: z.coerce.date(),
+    description: z.string(),
+    draft: z.boolean().optional().default(false),
+  }),
+});
+
+export const collections = { blog };