Create release notes page at torproject.org

On #29440 (moved) we discussed the possibility to have release notes per each browser release outside the blog. That is useful for different reasons; one of them is having a centralized place to look for these notes.

I'd suggest including this at

torproject.org/torbrowser/%version%/releasenotes torproject.org/torbrowserandroid/%version%/releasenotes

We may want to automate it in many ways, the first iteration could be static as well.