Pin the Rust toolchain for CI build jobs and let dependabot update it (#5938)

.github/workflows/docs.yaml

@@ -1,3 +1,4 @@
+# Copyright 2025, 2026 Element Creations Ltd.
 # Copyright 2025 New Vector Ltd.
 #
 # SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
@@ -30,8 +31,7 @@ jobs:
           persist-credentials: false
 
       - name: Install Rust toolchain
-        # Install the minimal toolchain, which includes rustc, rustdoc, and cargo
-        run: rustup toolchain install stable --profile minimal
+        run: rustup toolchain install
 
       - name: Setup sccache
         uses: mozilla-actions/sccache-action@9e7fa8a12102821edf02ca5dbea1acd0f89a2696 # v0.0.10