The rails/rails gemnasium page (https://gemnasium.com/rails/rails) is not helping much.
It lists 0 dependencies so there is not much to track. Also our gems mostly depend
on other gems we control. There is not much point in monitoring them.

The batch was introduced with #3909.

This has no effect on the API docs. The main API page is taken from:
    railties/RDOC_MAIN.rdoc

[ci skip]

Updating the global Rails README following work by @strzalek et al which extracted Action View to a separate gem, and also some other improvements.

Summary of changes:

* Reordered the paragraphs from View-Model-Controller to Model-Controller-View, based on how (IMHO) most people think about the structure and hierarchy of the framework and their code in it.
* Added references to ActionView's README in the _View_ paragraph
* Reworded the _Controller_ paragraph to briefly mentioned routing as handled by Action Dispatch, and that both Action Dispatch and Action Controller work together to form the request-handling Action Pack
* Removed paragraph saying Action Pack is coupled with Action View, as this is no longer the case
* Added brief references to Action Mailer and Active Support, since they are both major Rails components. Briefly mentioned in the _View_ paragraph that Action View is often used to generate the body of the email, in addition to the other typical of using to build a response body.

links were redirects

We tend to use Markdown for everything but the API, the
README is no longer used as the home page of the API
after 7a5aa720, so we can now migrate it.