docs: use UTF-8 for redirect generation

[meir-may]

Summary

• Use explicit UTF-8 encoding when reading the redirect HTML template
• Use explicit UTF-8 encoding when writing generated redirect pages
• Use explicit UTF-8 encoding when reading redirect URL mappings

Closes #7749

Verification

• python3 python/docs/redirects/redirects.py "$TMPDIR"
• python3 -m py_compile python/docs/redirects/redirects.py