Migrated `docs` scripts to GitHub actions (#5345)

Migrated the following scripts to GHA
- test-doc
- test-rustdoc
- build-rustdoc
- build-implementers-guide
- publish-rustdoc (only runs when `master` is modified)

Resolves paritytech/ci_cd#1016

---

Some questions I have:
- Should I remove the equivalent scripts from the `gitlab-ci` files?

---------

Co-authored-by: Alexander Samusev <41779041+alvicsam@users.noreply.github.com>

.github/workflows/docs.yml

+name: Docs
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review, labeled]
+  merge_group:
+
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+
+jobs:
+  set-image:
+    # GitHub Actions allows using 'env' in a container context.
+    # However, env variables don't work for forks: https://github.com/orgs/community/discussions/44322
+    # This workaround sets the container image for each job using 'set-image' job output.
+    # TODO: remove once migration is complete or this workflow is fully stable
+    if: contains(github.event.label.name, 'GHA-migration') || contains(github.event.pull_request.labels.*.name, 'GHA-migration')
+    runs-on: ubuntu-latest
+    outputs:
+      IMAGE: ${{ steps.set_image.outputs.IMAGE }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - id: set_image
+        run: cat .github/env >> $GITHUB_OUTPUT
+  test-rustdoc:
+    runs-on: arc-runners-polkadot-sdk-beefy
+    needs: [set-image]
+    container:
+      image: ${{ needs.set-image.outputs.IMAGE }}
+    steps:
+      - uses: actions/checkout@v4
+      - run: forklift cargo doc --workspace --all-features --no-deps
+        env:
+          SKIP_WASM_BUILD: 1
+  test-doc:
+    runs-on: arc-runners-polkadot-sdk-beefy
+    needs: [set-image]
+    container:
+      image: ${{ needs.set-image.outputs.IMAGE }}
+    steps:
+      - uses: actions/checkout@v4
+      - run: forklift cargo test --doc --workspace
+        env:
+          RUSTFLAGS: "-Cdebug-assertions=y -Dwarnings"
+
+  build-rustdoc:
+    runs-on: arc-runners-polkadot-sdk-beefy
+    needs: [set-image, test-rustdoc]
+    container:
+      image: ${{ needs.set-image.outputs.IMAGE }}
+    steps:
+      - uses: actions/checkout@v4
+      - run: forklift cargo doc --all-features --workspace --no-deps
+        env:
+          SKIP_WASM_BUILD: 1
+          RUSTDOCFLAGS: "-Dwarnings --default-theme=ayu --html-in-header ./docs/sdk/assets/header.html --extend-css ./docs/sdk/assets/theme.css --html-after-content ./docs/sdk/assets/after-content.html"
+      - run: rm -f ./target/doc/.lock
+      - run: mv ./target/doc ./crate-docs
+      - name: Inject Simple Analytics script
+        run: |
+          script_content="<script async defer src=\"https://apisa.parity.io/latest.js\"></script><noscript><img src=\"https://apisa.parity.io/latest.js\" alt=\"\" referrerpolicy=\"no-referrer-when-downgrade\" /></noscript>"
+          docs_dir="./crate-docs"
+
+          inject_simple_analytics() {
+            find "$1" -name '*.html' | xargs -I {} -P "$(nproc)" bash -c 'file="{}"; echo "Adding Simple Analytics script to $file"; sed -i "s|</head>|'"$2"'</head>|" "$file";'
+          }
+
+          inject_simple_analytics "$docs_dir" "$script_content"
+      - run: echo "<meta http-equiv=refresh content=0;url=polkadot_sdk_docs/index.html>" > ./crate-docs/index.html
+      - uses: actions/upload-artifact@v4
+        with:
+          name: ${{ github.sha }}-doc
+          path: ./crate-docs/
+          retention-days: 1
+          if-no-files-found: error
+
+  build-implementers-guide:
+    runs-on: ubuntu-latest
+    container:
+      image: paritytech/mdbook-utils:e14aae4a-20221123
+      options: --user root
+    steps:
+      - uses: actions/checkout@v4
+      - run: mdbook build ./polkadot/roadmap/implementers-guide
+      - run: mkdir -p artifacts
+      - run: mv polkadot/roadmap/implementers-guide/book artifacts/
+      - uses: actions/upload-artifact@v4
+        with:
+          name: ${{ github.sha }}-guide
+          path: ./artifacts/
+          retention-days: 1
+          if-no-files-found: error
+
+  publish-rustdoc:
+    if: github.ref == 'refs/heads/master'
+    runs-on: ubuntu-latest
+    environment: subsystem-benchmarks
+    needs: [build-rustdoc, build-implementers-guide]
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: gh-pages
+      - uses: actions/create-github-app-token@v1
+        id: app-token
+        with:
+          app-id: ${{ secrets.POLKADOTSDK_GHPAGES_APP_ID }}
+          private-key: ${{ secrets.POLKADOTSDK_GHPAGES_APP_KEY }}
+      - name: Ensure destination dir does not exist
+        run: |
+          rm -rf book/
+          rm -rf ${REF_NAME}
+        env:
+          REF_NAME: ${{ github.head_ref || github.ref_name }}
+      - name: Download rustdocs
+        uses: actions/download-artifact@v4
+        with:
+          name: ${{ github.sha }}-doc
+          path: ${{ github.head_ref || github.ref_name }}
+      - name: Download guide
+        uses: actions/download-artifact@v4
+        with:
+          name: ${{ github.sha }}-guide
+          path: /tmp
+      - run: mkdir -p book
+      - name: Move book files
+        run: mv /tmp/book/html/* book/
+      - name: Push to GH-Pages branch
+        uses: github-actions-x/commit@v2.9
+        with:
+          github-token: ${{ steps.app-token.outputs.token }}
+          push-branch: "gh-pages"
+          commit-message: "___Updated docs for ${{ github.head_ref || github.ref_name }}___"
+          force-add: "true"
+          files: ${{ github.head_ref || github.ref_name }}/ book/
+          name: devops-parity
+          email: devops-team@parity.io