add support for docs

[mihkeleidast]

This PR will:

• fix the usage for docs (the logic is now more similar to what happens in @frctl/handlebars)
• add documentaton for how to use this adapter properly for docs
• resolve Does not work for documentation entities #19

Background for the documentation weirdness:
If the engines are set in the opposite order (for components first, then for docs), then due to the fact how the adapter currently extends & monkeypatches Twig, all includes will try to reference docs entities instead of component entities.

I think we should try to find a way to improve that, since the order of setting engines in the config should not make a difference.

Manually tested that, when following the docs:

• including components via handles still works
• including components via full path (with the usage of base config option, i don't believe regular relative paths have ever worked)
• render tag will continue to work