Documentation revision / reorganization

[ebullient]

Joint effort with docs, engineering and community to revise docs for better readability / navigability.

In the linked PR, please review:

• _templates -- base for future docs
• _examples -- one-off examples used by docs explaining how to write docs
• doc-concepts.adoc -- skeleton, will eventually explain diataxis concepts
• doc-contribute-quarkus-docs-howto.adoc -- How-to guide explaining how to contribute to docs
• doc-create-tutorial.adoc -- A tutorial that describes creating a tutorial
• doc-reference.adoc -- Detailed reference for Quarkus Docs.