The sentence used to read "at the bottom of the the compiled file"

This reverts commit abf8de85.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.

irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]

This reverts commit bfafa999.

Changes in templates aren't allowed in docrails.

[ci skip]

You need to have @implicit_readonly set for this to happen,
and it's false by defualt.

Fixes #3386.

Mention that in addition to `rake routes` a developer can also visit `/rails/info/routes` in the browser.

This is still an early draft, but let's add it to the index
to have it ready.

- remove Palmist which is no longer available.
- update link to Query Reviewer.

[ci skip]

- remove query_stats plugin reference in debugging guide since it is no longer maintained.
- remove exception_logger plugin reference in debugging guide since it is no longer maintained.
- update query_reviewer url to the latest one on github.

This option is now unsupported in sprockets-rails.

The purpose of this refactor is twofold:

  * Emphasize the Rails development box as
    the recommended way to setup a dev
    environment by taking out the rest of
    the instructions.

  * Lighten the contributing guide, this
    non-essential information takes a
    disproportionate amount of space now
    that we have the rails-dev-box.