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/wikipedia.ex

@@ -1,6 +1,7 @@
 defmodule Wikipedia do
   alias Wikipedia.API
 
+  @spec get_artist_summary(String.t()) :: {:ok, map()} | {:error, :no_english_wikipedia | term()}
   def get_artist_summary(wikidata_id) do
     config = wikipedia_config()