Adds the full README (quickstart, interaction styles, privileged intents,
cache, REST, slash registration, event semantics, Nostrum porting guide,
reliability design), an examples/echo_bot.exs Mix.install script, ex_doc
config in mix.exs, and rewrites Dexcord.Gateway's moduledoc to describe
the finished reliability behavior instead of stale Phase 0-1 scope notes.
