Add ruby styleguide page to the dev guidelines

Add a new page to list specific styles that are not
covered by rubocop, specifically to help prevent
bikeshedding.

doc/development/backend/ruby_style_guide.md

+---
+type: reference, dev
+stage: none
+group: Development
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Ruby style guide
+
+This is a GitLab-specific style guide for Ruby code.
+
+Generally, if a style is not covered by [existing rubocop rules or styleguides](../contributing/style_guides.md#ruby-rails-rspec), it shouldn't be a blocker.
+Before adding a new cop to enforce a given style, make sure to discuss it with your team.
+When the style is approved by a backend EM or by a BE staff eng, add a new section to this page to
+document the new rule. For every new guideline, add it in a new section and link the discussion from the section's
+[version history note](../documentation/styleguide/index.md#version-text-in-the-version-history)
+to provide context and serve as a reference.
+
+Just because something is listed here does not mean it cannot be reopened for discussion.
+
+## Instance variable access using `attr_reader`
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52351) in GitLab 14.1.
+
+Instance variables can be accessed in a variety of ways in a class:
+
+```ruby
+# public
+class Foo
+  attr_reader :my_var
+
+  def initialize(my_var)
+    @my_var = my_var
+  end
+
+  def do_stuff
+    puts my_var
+  end
+end
+
+# private
+class Foo
+  def initialize(my_var)
+    @my_var = my_var
+  end
+
+  private
+
+  attr_reader :my_var
+
+  def do_stuff
+    puts my_var
+  end
+end
+
+# direct
+class Foo
+  def initialize(my_var)
+    @my_var = my_var
+  end
+
+  private
+
+  def do_stuff
+    puts @my_var
+  end
+end
+```
+
+Public attributes should only be used if they are accessed outside of the class.
+There is not a strong opinion on what strategy is used when attributes are only
+accessed internally, as long as there is consistency in related code.

doc/development/contributing/style_guides.md

@@ -139,7 +139,10 @@ reviewers/maintainers must not ask authors to use one style or the other, as bot
 are accepted. This isn't an ideal situation since this leaves space for
 [bike-shedding](https://en.wiktionary.org/wiki/bikeshedding), and ideally we
 should enable all RuboCop rules to avoid style-related
-discussions/nitpicking/back-and-forth in reviews.
+discussions/nitpicking/back-and-forth in reviews. There are some styles that
+commonly come up in reviews that are not enforced, the
+[GitLab Ruby style guide](../backend/ruby_style_guide.md) includes a non-exhaustive
+list of these topics.
 
 Additionally, we have a dedicated
 [newlines style guide](../newlines_styleguide.md), as well as dedicated