docs: Generate document with utf8 encoding

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26515>

bin/gen_release_notes.py

@@ -331,7 +331,7 @@ def update_release_notes_index(version: str) -> None:
             new_relnotes.append(f'   {version} <relnotes/{version}>\n')
         new_relnotes.append(line)
 
-    with relnotes_index_path.open('w') as f:
+    with relnotes_index_path.open('w', encoding='utf-8') as f:
         for line in new_relnotes:
             f.write(line)
 
@@ -357,7 +357,7 @@ async def main() -> None:
     )
 
     final = pathlib.Path('docs') / 'relnotes' / f'{this_version}.rst'
-    with final.open('wt') as f:
+    with final.open('wt', encoding='utf-8') as f:
         try:
             f.write(TEMPLATE.render(
                 bugfix=is_point_release,