Shorten unnecessarily long names for Guides

The names are only used in the Guides Index overlay and several of them are
longer than they need to be.

For example “Ruby on Rails” is mentioned over and over again in the Release
Notes section although these are obviously all Rails versions we’re listing.

There’s a Maintenance Policy category with a Maintenance Policy article which
is redundant. Policies makes much more sense as a category.

Similarly, Contributing to Ruby on Rails is both a category and an article.
“Contributions” is a better category title. “Ruby on Rails Guides Guidelines” is
a gigantic mouthful. We are *in* the Rails Guides, we don’t need to specify
what guides we’re talking about here.

As a final added advantage, all of these shortenings make the index much easier
to parse in order to find an article.

[ci skip]

guides/source/documents.yaml

@@ -173,7 +173,7 @@
       description: This guide describes the considerations needed and tools available when working directly with concurrency in a Rails application.
       work_in_progress: true
 -
-  name: Contributing to Ruby on Rails
+  name: Contributions
   documents:
     -
       name: Contributing to Ruby on Rails
@@ -184,14 +184,14 @@
       url: api_documentation_guidelines.html
       description: This guide documents the Ruby on Rails API documentation guidelines.
     -
-      name: Ruby on Rails Guides Guidelines
+      name: Guides Guidelines
       url: ruby_on_rails_guides_guidelines.html
       description: This guide documents the Ruby on Rails guides guidelines.
 -
-  name: Maintenance Policy
+  name: Policies
   documents:
     -
-      name: Maintenance Policy for Ruby on Rails
+      name: Maintenance Policy
       url: maintenance_policy.html
       description: What versions of Ruby on Rails are currently supported, and when to expect new versions.
 -
@@ -202,51 +202,51 @@
       url: upgrading_ruby_on_rails.html
       description: This guide helps in upgrading applications to latest Ruby on Rails versions.
     -
-      name: Ruby on Rails 6.0 Release Notes
+      name: 6.0 Release Notes
       work_in_progress: true
       url: 6_0_release_notes.html
       description: Release notes for Rails 6.0.
     -
-      name: Ruby on Rails 5.2 Release Notes
+      name: 5.2 Release Notes
       url: 5_2_release_notes.html
       description: Release notes for Rails 5.2.
     -
-      name: Ruby on Rails 5.1 Release Notes
+      name: 5.1 Release Notes
       url: 5_1_release_notes.html
       description: Release notes for Rails 5.1.
     -
-      name: Ruby on Rails 5.0 Release Notes
+      name: 5.0 Release Notes
       url: 5_0_release_notes.html
       description: Release notes for Rails 5.0.
     -
-      name: Ruby on Rails 4.2 Release Notes
+      name: 4.2 Release Notes
       url: 4_2_release_notes.html
       description: Release notes for Rails 4.2.
     -
-      name: Ruby on Rails 4.1 Release Notes
+      name: 4.1 Release Notes
       url: 4_1_release_notes.html
       description: Release notes for Rails 4.1.
     -
-      name: Ruby on Rails 4.0 Release Notes
+      name: 4.0 Release Notes
       url: 4_0_release_notes.html
       description: Release notes for Rails 4.0.
     -
-      name: Ruby on Rails 3.2 Release Notes
+      name: 3.2 Release Notes
       url: 3_2_release_notes.html
       description: Release notes for Rails 3.2.
     -
-      name: Ruby on Rails 3.1 Release Notes
+      name: 3.1 Release Notes
       url: 3_1_release_notes.html
       description: Release notes for Rails 3.1.
     -
-      name: Ruby on Rails 3.0 Release Notes
+      name: 3.0 Release Notes
       url: 3_0_release_notes.html
       description: Release notes for Rails 3.0.
     -
-      name: Ruby on Rails 2.3 Release Notes
+      name: 2.3 Release Notes
       url: 2_3_release_notes.html
       description: Release notes for Rails 2.3.
     -
-      name: Ruby on Rails 2.2 Release Notes
+      name: 2.2 Release Notes
       url: 2_2_release_notes.html
       description: Release notes for Rails 2.2.