Special characters in ngff-spec lead to build errors

[jo-mueller]

I was just just working with @cwetzker on compiling the docs locally and we ran into some unicode decode errors due to some special characters in the ngff-spec repo. Those appear on every spec branch in the build_footer function. There, the © and ® characters are used which are leading to trouble down the road.

Somehow it doesn't appear on every plattform (my machine doesn't complain), but there's probably a safer way to express these symbols.

[lubianat]

@jo-mueller do you have more details on the settings where you did see the error? just for reference

[jo-mueller]

I couldn't reproduce it on my machine and forgot to look at the precise environment setup. But replacing these characters with ® and © solved it.