Add all official adapter docs to the docs site

[mihkeleidast]

Currently, the main docs site only features Handlebars adapter docs. All other official adapter docs exist only in their respective package README files.

This is quite limiting, because:

• Exposing only Handlebars as the "default" adapter in the docs site and the code examples there is not the way forward, because Handlebars as a templating language is very limited. Additionally, since adapters differ in their functionality (bundled helpers/filters etc), it should be more clear that the current docs are only for Handlebars and what the alternatives are.
• The visibility of other adapters is not that great, since they are not exposed in the docs site aside from a few links here and there.
• Single README file approach for other adapters is very limited. Some things should be split apart and described further. But this is very hard to do in a single file.

TODO list:

• [ ] figure out the best structure for documenting multiple adapters and migrate the current Handlebars docs to that structure
• [ ] add nunjucks adapter docs
• [ ] add twig adapter docs
• [ ] add react adapter docs