refactors and fixes in guides generation [ci skip]

This commit is not precisely atomic, but the changes have evolved, summary:

* The ENV-based interface has been moved upwards, the generator has now a conventional
  initializer.

* RAILS_VERSION is now assumed to be a Git tag. A blank RAILS_VERSION means edge guides.

* In consequence, the EDGE env variable is gone.

* The "local" version is also gone, the current SHA1 is computed for edge guides.

* Assumes guides are generated from a repo checkout (time ago users could
  generate them from gems.)

* The WARNINGS flag is gone in consequence, you cannot disable warnings.

* The `api_link` Markdown helper is fixed.

* Docs about usage have one single place: rake guides:help.

* Links in guides have been revised.

1 files changed, 9 insertions, 6 deletions

diff --git a/guides/rails_guides/markdown.rb b/guides/rails_guides/markdown.rb
index 009e5aff99..bf2cc82c7c 100644
--- a/guides/rails_guides/markdown.rb
+++ b/guides/rails_guides/markdown.rb
@@ -4,12 +4,14 @@ require "rails_guides/markdown/renderer"
 
 module RailsGuides
   class Markdown
-    def initialize(view, layout)
-      @view = view
-      @layout = layout
+    def initialize(view:, layout:, edge:, version:)
+      @view          = view
+      @layout        = layout
+      @edge          = edge
+      @version       = version
       @index_counter = Hash.new(0)
-      @raw_header = ""
-      @node_ids = {}
+      @raw_header    = ""
+      @node_ids      = {}
     end
 
     def render(body)
@@ -60,7 +62,8 @@ module RailsGuides
           autolink: true,
           strikethrough: true,
           superscript: true,
-          tables: true)
+          tables: true
+        )
       end
 
       def extract_raw_header_and_body