First pass at uniformed types, specs and docs

- spec public functions (skipping controllers, views, live views and
components)
- use types instead of explanations in docs
- remove redundant docs
- fix typos

lib/last_fm/scrobble.ex

@@ -1,6 +1,16 @@
 defmodule LastFm.Scrobble do
   defstruct [:track, :artist, :timestamp, :album, :album_artist, :mbid]
 
+  @type t :: %__MODULE__{
+          track: String.t(),
+          artist: String.t(),
+          timestamp: integer(),
+          album: String.t() | nil,
+          album_artist: String.t() | nil,
+          mbid: String.t() | nil
+        }
+
+  @spec encode(t()) :: map()
   def encode(scrobble) do
     scrobble
     |> Map.from_struct()