Minor Forum Formatting issues

[hacatu]

On the forum, I've noticed a couple small issues:

• nested collapsable elements, like `code` within `collapse`, do not expand the outer element when the inner element expands, so some contents can get cut off
• the C/C++ language highlighting breaks with C 2023 digit separators in numbers, ie if you write 1'000, it will parse it as the start of an unterminated character literal and break highlighting on the rest of the program