Improve "quickstart dune project" page

[bluddy]

Update it to use the new dune init.

Also (from discord):

• how to init a new project (this seems to be dune init proj , for example.. But should I first create an opam switch ? )
• how to add a dependency, have it fetched and be able to use it within e.g. files in lib//bin/ in case of a dune project (is adding it to dune-project enough ? how is it fetched? as part of dune-build? No? Then what?)
• what to version, what to ignore (e.g. add _opam to .gitignore, it's generated from using ocaml swich)
• how OTHERS can clone and setup their dev environment from my existing project.