Fix documentation on including templates, add mzdoc template to documentation

[pepe]

As you suggested in the #9, I have added the template-include fn. I defined it as a convenience function in the ast as it seemed like the most straightforward solution. I changed the documentation from :template key to this new fn.

I also added the mzdoc template to the list of built-in templates.

[pepe]

After some thought, it finally clicked for me, that we do not need fn/macro, but we can use require straight in the parent template.

I removed the template-include fn and fixed the docs.

[bakpakin]

Looks good, thanks.