Better docs

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Nicer docs

No 3rd stage

Add mix.deps get before docs

Add :ex_doc on test env so that it runs into CI

support/guides/development/styleguide.md

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+# Styleguide
+
+# Elixir
+
+We format our code with the Elixir Formatter and check for issues with [Credo](https://github.com/rrrene/credo) (a few rules are not blocking).
+
+Please run those two commands before pushing code :
+ * `mix format`
+ * `mix credo`
+ 
+These two commands must not return an error code, since they are required to pass inside CI.
+
+# Front
+
+We use `tslint` with the `tslint-config-airbnb` preset.
+Errors should be reported when running in dev mode `npm run dev` or when building a production bundle `npm run build`.