Documentation: Ensure HTML files are UTF-8 encoded

[LukeSerne]

This is a follow-up of #5462. Unlike that PR, this PR fixes the issue in the source files and regenerates the .html files. The cause of the issue turned out to be the chunking style defaulting to ISO-8859-1 encoding. This PR changes the encoding to UTF-8, which makes the resulting files easier to edit and the diffs generated by github easier to read.