docs: use utf-8 for redirect generation

[Karunasagar12]

Summary

• Uses explicit encoding="utf-8" when reading the redirect template
• Uses explicit encoding="utf-8" when reading redirect URL mappings
• Uses explicit encoding="utf-8" when writing generated redirect pages

Fixes #7749

Test Plan

• python3 -m py_compile python/docs/redirects/redirects.py
• Ran python3 python/docs/redirects/redirects.py "$tmp"
• Verified 341 redirect index.html pages were generated and a sample page could be read as UTF-8