ci: simplify CI workflow and upgrade cpp-actions to @develop

Author: alandefreitas

.github/llvm-matrix.js

@@ -0,0 +1,61 @@
+#!/usr/bin/env bash
+#
+# Licensed under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+# Copyright (c) 2026 Alan de Freitas (alandefreitas@gmail.com)
+#
+# Official repository: https://github.com/cppalliance/mrdocs
+#
+
+# Compare current demos against previous develop demos.
+# Expected env vars: GITHUB_OUTPUT
+set -x
+
+LOCAL_DEMOS_DIR="./demos/"
+PREV_DEMOS_DIR="./demos-previous/"
+DIFF_DIR="./demos-diff/"
+
+if [[ ! -d $PREV_DEMOS_DIR || -z $(ls -A $PREV_DEMOS_DIR) ]]; then
+    echo "No previous demos found."
+    echo "diff=false" >> "$GITHUB_OUTPUT"
+    exit 0
+fi
+
+mkdir -p $PREV_DEMOS_DIR $DIFF_DIR
+
+find $PREV_DEMOS_DIR -type f | while read previous_file; do
+    local_file="${LOCAL_DEMOS_DIR}${previous_file#$PREV_DEMOS_DIR}"
+    diff_output="$DIFF_DIR${previous_file#$PREV_DEMOS_DIR}"
+    if [[ -f $local_file ]]; then
+        mkdir -p "$(dirname "$diff_output")"
+        diff "$previous_file" "$local_file" > "$diff_output"
+        if [[ ! -s $diff_output ]]; then
+            rm "$diff_output"
+        fi
+    else
+        echo "LOCAL FILE $local_file DOES NOT EXITS." > "$diff_output"
+        echo "PREVIOUS CONTENT OF THE FILE WAS:" >> "$diff_output"
+        cat "$previous_file" >> "$diff_output"
+    fi
+done
+
+find $LOCAL_DEMOS_DIR -type f | while read local_file; do
+    previous_file="${PREV_DEMOS_DIR}${local_file#$LOCAL_DEMOS_DIR}"
+    diff_output="$DIFF_DIR${local_file#$LOCAL_DEMOS_DIR}"
+    if [[ ! -f $previous_file ]]; then
+        echo "PREVIOUS $previous_file DOES NOT EXIST." > "$diff_output"
+        echo "IT HAS BEEN INCLUDED IN THIS VERSION." >> "$diff_output"
+        echo "NEW CONTENT OF THE FILE IS:" >> "$diff_output"
+    fi
+done
+
+if [[ -z $(ls -A $DIFF_DIR) ]]; then
+    echo "No differences found."
+    echo "diff=false" >> "$GITHUB_OUTPUT"
+else
+    N_FILES=$(find $DIFF_DIR -type f | wc -l)
+    echo "Differences found in $N_FILES output files."
+    echo "diff=true" >> "$GITHUB_OUTPUT"
+fi