docs: prepare for stabilization

Describe the package in terms of its actual role today, and note its current level of stability and usage.
Explicitly note that this is the future via RFC 0779.
Add full installation and setup instructions, with links as appropriate to other tools in the ecosystem.
Make all the examples match what Prettier does with it.
Move the discussion of hbs to a new 'History' section.

ember-cli / ember-template-imports