Add Gitbook documentation to target info

Hi
I have been working on documentation for different target for some time now:

- [haxe and JavaScript](https://github.com/MatthijsKamstra/haxejs)
- [haxe and PHP](https://github.com/MatthijsKamstra/haxephp)
- [haxe and nodejs](https://github.com/MatthijsKamstra/haxenode)

In the HaxeManual most of the targets are not that well documented (take [PHP](https://haxe.org/manual/target-php.html) for example), @markknol also made some issues related to this (#139, #138, #137, #136, #135 opened on 12 Apr 2015)

Because documentation writing is difficult, I thought it would be a good idea to add these gitbooks to the official documentation.

For example: https://haxe.org/manual/target-php.html.
Add a chapter "community driven documentation" with some disclaimers like "bla bla bla, community, bla, not maintained by Haxe developers team but friendly developers, bla ... link"

I can transfer the repos if needed

Let me know what you think!

Grt Matthijs