Replace bind() with on() as suggested by the JQuery bind() documentation: https://api.jquery.com/bind/

* Add form_tag generated output.
* improve some text.
* data-remote='true' should use double quotes.

`a[data-color]` is not used in example html.

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

Talked with @fxn about this. Bullet points should have periods at the ends.

We discussed this on Twitter, seems to be the best option.

I believe the topics and their headlines should be consistent for obvious reasons. Also

Originally, this guide was called "AJAX on Rails," but really, it's
not just about AJAX. This was never finished, but it got accidentally
generated and Google found out about it.

In the meantime, all the guides were converted to markdown, as well.

So here's a new guide. It covers all of the built-in helpers that
use ajax requests, it covers CoffeeScript, and it covers UJS.