fix(docs): use explicit UTF-8 encoding in redirect generation script

[Xx (Xx-173)]

Summary

Fixes #7749. python/docs/redirects/redirects.py reads and writes redirect files using Python's default system encoding. On Windows locales where the default is not UTF-8 (e.g. cp950), this fails with UnicodeDecodeError/UnicodeEncodeError when the files contain non-ASCII characters.

Added explicit encoding="utf-8" to all three file operations:

• reading the HTML redirect template (HTML_PAGE_TEMPLATE_FILE.open("r", encoding="utf-8"))
• writing generated index.html (open(redirect_page_path, "w", encoding="utf-8"))
• reading redirect_urls.txt (open(REDIRECT_URLS_FILE, "r", encoding="utf-8"))

This is purely a docs-tooling portability fix; no runtime behavior change for UTF-8 environments.

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