[boost] Main page should link to some online compilers that include Boost

Similar to https://isocpp.org/blog/2013/01/online-c-compilers but less of a blog post and more of an up-to-date list of online compilers where users can try out Boost immediately. Googling for this is tedious and it's certainly in Boost's best interest to encourage people to try it out (and make the barrier to do so as low as possible). -Matt