Add required documentation for GCC release

[CohenArthur]

• Add changes to changes.html (https://gcc.gnu.org/gcc-13/changes.html)
• Contribute to gcc.gnu.org/wiki for some explanations on how to use the compiler
• Mention the state of the compiler in changes.html
• Add documentation about our compiler's internal
• More documentation needed?

[tschwinge]

That's what I said in yesterday's "gccrs weekly" meeting that I'll take care of. 👍

[CohenArthur]

That's what I said in yesterday's "gccrs weekly" meeting that I'll take care of. +1

Yes! And thank you so much 💜 But I didn't want to just write "Thomas said he would do it", that seemed a little too harsh for me haha