How to: Override devise default routes for sign in and sign out

In one of my application, I need to override default routes provided by Devise for better SEO.

SEO expert asked me to change default url in such way,
"/users/sign_in" => "/sign-in"
"/users/sign_up" => "/sign-up"

And I know, it is possible in devise. Devise is awesome authentication gem which provides lots of customization as per our app needs. In fact, every gem publisher should study the devise code for better understanding on how to provide api's for customization.

So here is the full customization of default routes,

redirect section : 
        On line 5 and 6, I wrote a redirection rule for SEO purpose and for demonstration over here. But best place is to place redirection rules in your web server, that is in nginx or apache.
        This only applies to you, if your app is already live and search engine has already indexed your sign in and sign out routes.

skip section :
        On line 10, I have skipped generation of default routes for session(sign_in) and registration(sign_up).

devise_scope block :
        In devise_scope block, we need to provide our own urls for sign_in and sign_out. So we need to provide url format with controller and action.
        Also, for each urls, we need to provide path helper using ":as", so that we can still use old path name, without code change.