chore: report total commits by category in release notes (#6931)

This PR reports a sentence like:

```quote
For this release, 201 changes landed. In addition to the 74 feature additions and 44 fixes listed below there were 7 refactoring changes, 5 documentation improvements and 62 chores.
```

in the automatically generated release notes.

script/release_notes.py

@@ -65,6 +65,24 @@ def format_markdown_description(pr_number, description):
     link = f"[#{pr_number}](https://github.com/leanprover/lean4/pull/{pr_number})"
     return f"{link} {description}"
 
+def count_commit_types(commits):
+    counts = {
+        'total': len(commits),
+        'feat': 0,
+        'fix': 0,
+        'refactor': 0,
+        'doc': 0,
+        'chore': 0
+    }
+    
+    for _, first_line, _ in commits:
+        for commit_type in ['feat:', 'fix:', 'refactor:', 'doc:', 'chore:']:
+            if first_line.startswith(commit_type):
+                counts[commit_type.rstrip(':')] += 1
+                break
+    
+    return counts
+
 def main():
     parser = argparse.ArgumentParser(description='Generate release notes from Git commits')
     parser.add_argument('--since', required=True, help='Git tag to generate release notes since')
@@ -132,6 +150,13 @@ def main():
         for label in changelog_labels:
             changelog[label].append((pr_number, markdown_description))
 
+    # Add commit type counting
+    counts = count_commit_types(commits)
+    print(f"For this release, {counts['total']} changes landed. "
+          f"In addition to the {counts['feat']} feature additions and {counts['fix']} fixes listed below "
+          f"there were {counts['refactor']} refactoring changes, {counts['doc']} documentation improvements "
+          f"and {counts['chore']} chores.\n")
+
     section_order = sort_sections_order()
     sorted_changelog = sorted(changelog.items(), key=lambda item: section_order.index(format_section_title(item[0])) if format_section_title(item[0]) in section_order else len(section_order))