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/api/signature.ex

@@ -1,4 +1,5 @@
 defmodule LastFm.API.Signature do
+  @spec generate(map() | keyword(), String.t()) :: String.t()
   def generate(params, shared_secret) do
     encoded_params =
       params