Updates all copyright notices to include "Ghostty contributors" to
reflect the fact that Mitchell is not the sole copyright owner.
Also adds "Ghostty contributors" to the author section in the manpages,
linking https://github.com/ghostty-org/ghostty/graphs/contributors for
proper credit.
This is my third (!) attempt at implementing localization support.
By leveraging GTK builder to do most of the `gettext` calls, I
can avoid the whole mess about missing symbols on non-glibc platforms.
Added some documentation too for contributors and translators,
just for good measure.
