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.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

The main goal is to improve readability, but as a side effect this commit makes the method a bit faster.

Some translations of rails guides has their own documents.yaml, for example

Japanese:
https://github.com/yasslab/railsguides.jp/blob/japanese/guides/source/documents.yaml

Traditional Chinese:
https://github.com/docrails-tw/guides/blob/master/rails_guides/helpers.rb#L21,L27
https://github.com/docrails-tw/guides/blob/master/source/documents_zh-TW.yaml

The [Translating Rails Guides](https://github.com/rails/docrails/wiki/translating-rails-guides) says

> Copy the contents of guides/source into your own language directory and translate them.

So that sounds ok to copy guides/source/documents.yaml into language
specific directly and translating them like other markdown files.

Shelling out was there for authors convenience, but we are
rather going to have the tag or SHA1 always in RAILS_VERSION
and if the environment variable is blank, then just use
"local" as a reminder that you are just working locally.

The docs server has been updated to set the long SHA1 in
RAILS_VERSION when generating edge guides.

This reverts commit 09682e9e.

Conflicts:
	guides/source/en/association_basics.md

REASON: Translation work can not be done in the docrails repository

[guides] guides:generate:kindle is the new interface to generate .mobi file instead of using KINDLE=1

[guides] get rid of any reference to the generator script since rake tasks are now the new interface

* Passing edge information to the views fixes the lack of the edge badge
and sets the appropriate welcome page on edgeguides.rubyonrails.org.
* Changes in the welcome message to hyper link the rails tree based on
the version.
* Mention the Amazon url from where kindlegen can be downloaded.

in the template name while generating guide related pages
like index, layout and credits

Normally I would have done this in master, but there was already
a guide with wrapped content, so worked here. I am going to
cross-merge now.

Reason: <notextile> is an internal tag

This reverts commit 01040b5d.

This reverts commit 3e6052a4.

Reason: Code changes are not allowed in docrails (even if they are related to guides generation)

[GUIDES] Changed the `LANGUAGE` variable to `GUIDES_LANGUAGE` to minimize conflicts with regular environment variables (eg. in Ubuntu)

[FIX] Renamed ENV['LANG'] for generating translated guides to ENV['LANGUAGE'] so there's no mixup with OS env vars

Added support for generating translated guides (stored in `source/<LANG>`) via setting the LANG environment variable