Add initial "templates" doc.

[mtias]

Closes #4340.

[mtias]

Let's iterate on this.

[BinaryMoon]

Hi Matias. Looks good to me.

A few things I think would be good to include are:

1. How to add a template to an existing post type (such as posts/ jetpack testimonials). I did this in my starter theme here but don't know if it's the best way: https://github.com/BinaryMoon/granule/blob/master/inc/gutenberg.php#L68-L99

2. A note on how to find out what blocks are available. @youknowriad mentioned 'wp.blocks.getBlockTypes()' in Docs - Block Templates #4340 which is very helpful. I think if you want people outside of the core developers to start using/ implementing Gutenberg features then things like this need to be as visible as possible. Possibly this could be added to the blocks api as well/ instead?