docs: use explicit utf-8 encoding in redirects script

[wkd09]

Why are these changes needed?

This PR updates the docs redirect generation script to use explicit UTF-8 encoding when reading and writing text files.

Currently, python/docs/redirects/redirects.py relies on Python's default system encoding when:

• reading redirect_template.html
• reading redirect_urls.txt
• writing generated redirect index.html files

This can be less portable on environments where the default encoding is not UTF-8, such as some Windows locales.

Related issue number

Closes #7749

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Local check:

python -m py_compile python/docs/redirects/redirects.py```