Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

doc/api/examples/ruby/example_2.rb

@@ -41,7 +41,6 @@ next_cursor = nil
 proposals   = []
 
 loop do
-
   puts "> Requesting page #{page_number}"
 
   query = build_query(page_size: 25, page_number: page_number, next_cursor: next_cursor)