docs(site): add the Diátaxis documentation site

.github/workflows/docs.yml

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+name: Docs
+
+on:
+  push:
+    branches: ["main"]
+    paths:
+      - "docs-site/**"
+      - ".github/workflows/docs.yml"
+  pull_request:
+    paths:
+      - "docs-site/**"
+      - ".github/workflows/docs.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: withastro/action@v6
+        with:
+          path: "./docs-site"
+
+  deploy:
+    if: github.event_name == 'push'
+    needs: build
+    runs-on: ubuntu-latest
+    permissions:
+      pages: write
+      id-token: write
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v5

docs-site/.gitignore

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+node_modules/
+dist/
+.astro/

docs-site/astro.config.mjs

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+import starlight from "@astrojs/starlight";
+import { defineConfig } from "astro/config";
+
+export default defineConfig({
+  site: "https://aojdevstudio.github.io/Finance-Guru",
+  base: "/Finance-Guru/",
+  integrations: [
+    starlight({
+      title: "Finance Guru",
+      description: "Finance Guru is a local-first financial analysis and automation repository built for a private family office workflow. It combines a typed Python analysis engine, brokerage-data integrations, operational runbooks, and AI-assisted workflows.",
+      favicon: "/favicon.svg",
+      customCss: ["./src/styles/aoj-docs.css"],
+      lastUpdated: true,
+      sidebar: [
+        {
+          label: "Tutorials",
+          items: [{ autogenerate: { directory: "tutorials" } }],
+        },
+        {
+          label: "How-to guides",
+          items: [{ autogenerate: { directory: "how-to" } }],
+        },
+        {
+          label: "Reference",
+          items: [{ autogenerate: { directory: "reference" } }],
+        },
+        {
+          label: "Explanation",
+          items: [{ autogenerate: { directory: "explanation" } }],
+        },
+        {
+          label: "Project",
+          items: [{ label: "Source repository", link: "https://github.com/AojdevStudio/Finance-Guru" }],
+        },
+      ],
+    }),
+  ],
+});