feat: docs website (#80)

Deploy the documentation to GitHub Pages using `mdbook` and a little
more content for it. Can be visited at https://apricote.github.io/hcloud-upload-image.

.github/actions/setup-mdbook/action.yaml

@@ -0,0 +1,16 @@
+name: "Setup mdbook"
+inputs:
+  version:
+    description: "mdbook version"
+
+runs:
+  using: composite
+  steps:
+    - name: Setup mdbook
+      shell: bash
+      env:
+        url: https://github.com/rust-lang/mdbook/releases/download/${{ inputs.version }}/mdbook-${{ inputs.version }}-x86_64-unknown-linux-gnu.tar.gz
+      run: |
+        mkdir mdbook
+        curl -sSL "$url" | tar -xz --directory=./mdbook
+        echo `pwd`/mdbook >> $GITHUB_PATH

.github/workflows/docs.yaml

@@ -0,0 +1,39 @@
+name: docs
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write # To push a branch
+      pages: write # To push to a GitHub Pages site
+      id-token: write # To update the deployment status
+
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+        with:
+          lfs: "true"
+
+      - uses: ./.github/actions/setup-mdbook
+        with:
+          version: v0.4.42 # renovate: datasource=github-releases depName=rust-lang/mdbook
+
+      - name: Build Book
+        working-directory: docs
+        run: mdbook build
+
+      - name: Setup Pages
+        uses: actions/configure-pages@983d7736d9b0ae728b81ab479565c72886d7745b # v5
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@56afc609e74202658d3ffba0e8f6dda462b719fa # v3
+        with:
+          # Upload entire repository
+          path: "docs/book"
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@d6db90164ac5ed86f2b6aed7e0febac5b3c0c03e # v4