Merge the lists of JSX integrations in the docs.

There were these two lists of JSX tools at [Complimentary Tools](http://facebook.github.io/react/docs/complementary-tools.html#jsx-integrations)
and [Tooling Integration](http://facebook.github.io/react/docs/tooling-integration.html#helpful-open-source-projects), that were a bit out of sync with each other.

Bring them together and add a link to the former from the latter.