When generating documents, specify the encoding as UTF-8 when reading…

[OmegaZhou]

Description

When I try to build a document by myself, because the default encoding of my computer is not UTF-8, the document building fails. Therefore, I explicitly specify the encoding as UTF-8 when codes use the open statement to read and write files

Checklist:

Please make sure to complete this checklist before requesting a review.

• [x] My code follows the style guidelines of this project
• [x] My changes generate no new warnings
• [ ] My code also compiles for gpu_* and packet_* variants. If you can't test this, please leave below
• [ ] I have commented my code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I cleaned the commit history and removed any "Merge" commits
• [x] I give permission that the Mitsuba 2 project may redistribute my contributions under the terms of its license

[Speierers]

Thanks @OmegaZhou, I will merge this and port it on the Mitsuba 3 repository which will be released soon.