feat: add Jekyll website for Seahorse

- Add docs/_config.yml with Jekyll configuration
- Add docs/_layouts/default.html with dark theme
- Add docs/index.html with promotional content
- Add docs/Gemfile for Jekyll dependencies
- Add .github/workflows/jekyll.yml for GitHub Pages deployment

Author: CSL

.github/workflows/jekyll.yml

@@ -0,0 +1,55 @@
+name: Deploy Jekyll site to Pages
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+          bundler-cache: true
+
+      - name: Install dependencies
+        run: |
+          cd docs
+          bundle install
+
+      - name: Build Jekyll site
+        run: |
+          cd docs
+          bundle exec jekyll build
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: docs/_site
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

docs/Gemfile

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+# gem "rails"
+
+gem "jekyll", "~> 4.4"

docs/_config.yml

@@ -0,0 +1,15 @@
+title: Seahorse
+description: An elegant macOS material collection and management tool
+baseurl: "/Seahorse"
+url: "https://ssbun.github.io"
+
+# Build settings
+markdown: kramdown
+
+# Exclude unnecessary files
+exclude:
+  - Gemfile
+  - Gemfile.lock
+  - node_modules
+  - vendor/
+  - .git/