feat(docs,ci): enable docs build and publishing for the mainnet branch (ethereum#2775)

Co-authored-by: Sam Wilson <57262657+SamWilsn@users.noreply.github.com>

Author: danceratopz

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

@@ -0,0 +1,29 @@
+name: Setup uv and just
+description: Install uv, Python, and just for CI jobs
+inputs:
+  python-version:
+    description: Python version to install (e.g. "3.14", "pypy3.11")
+    default: "3.13"
+  enable-cache:
+    description: Enable uv dependency cache
+    default: "true"
+  cache-dependency-glob:
+    description: Glob for cache invalidation
+    default: "uv.lock"
+runs:
+  using: "composite"
+  steps:
+    - name: Install uv and python ${{ inputs.python-version }}
+      uses: astral-sh/setup-uv@e06108dd0aef18192324c70427afc47652e63a82 # v7.5.0
+      with:
+        enable-cache: ${{ inputs.enable-cache }}
+        cache-dependency-glob: ${{ inputs.cache-dependency-glob }}
+        version: "0.10.4"
+        python-version: ${{ inputs.python-version }}
+    - name: Prefer uv-managed Python over system Python
+      shell: bash
+      run: echo "UV_PYTHON_PREFERENCE=only-managed" >> "$GITHUB_ENV"
+    - name: Install just
+      uses: taiki-e/install-action@e24b8b7a939c6a537188f34a4163cb153dd85cf6 # v2.69.1
+      with:
+        tool: just@1.46

.github/configs/docs-branches.yaml

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+# Branches whose documentation is built and published to
+# steel.ethereum.foundation/docs/execution-specs/
+#
+# The `branches[]` list must stay in sync with steel-website's
+# `docs-config.yml`; a branch that publishes here but isn't listed there
+# is dispatched but dropped by the aggregator.
+#
+# Each entry defines:
+#   - path: The branch name (must match the Git branch exactly)
+#   - label: Human-readable label for the version switcher UI
+
+branches:
+  - path: "mainnet"
+    label: "Mainnet (BPO2)"
+  - path: "forks/amsterdam"
+    label: "Amsterdam"
+  - path: "devnets/bal/4"
+    label: "bal-devnet-4"