Doc drift: embedded mode wrongly documented as default; README shows removed API

[gsmlg]

Problems

1. Caddy.Server.Embedded moduledoc says embedded is used "when config :caddy, mode: :embedded (default)" (lib/caddy/server/embedded.ex:6) — but the actual default is :external (lib/caddy/config.ex:180).

2. README still documents the removed API:

   • Caddy.Config.set_bin("/usr/bin/caddy") → now Caddy.set_bin/1 (Config is a pure struct module, the Agent is Caddy.ConfigProvider)
   • No mention of the external/embedded mode split, the 3-part config model, sync_to_caddy/0, or the state machine (Caddy.get_state/0, Caddy.ready?/0)

README is the hexdocs landing page (docs: main: "readme"), so this is the first thing users see.