Add encoding when opening files in generate_mkdocs.py (#10244)
## Summary
Open files with utf8 encoding when writing files in generate_mkdocs.py.
The following can happen otherwise.
```
../ruff> python scripts/generate_mkdocs.py
Finished dev [unoptimized + debuginfo] target(s) in 0.25s
Running `target\debug\ruff_dev.exe generate-docs`
Traceback (most recent call last):
File "C:\..\ruff\scripts\generate_mkdocs.py", line 185, in <module>
main()
File "C:\..\scripts\generate_mkdocs.py", line 141, in main
f.write(clean_file_content(file_content, title))
File "C:\..\AppData\Local\Programs\Python\Python310\lib\encodings\cp1252.py", line 19, in encode
return codecs.charmap_encode(input,self.errors,encoding_table)[0]
UnicodeEncodeError: 'charmap' codec can't encode characters in position 1396-1397: character maps to <undefined>
```
I could not determine which character was causing the issue, but opening
with utf8 encoding fixed it.
## Test Plan
Condering the change is small, I simply ran the file and confirmed it
worked, but opened to suggestion on more robust testing.
