(Fixed typo from previous commit: ccd0b511)

Added notice on how to enable gzip compression in rails app using Rack::Deflater. it is disabled & commented out by default. 

Remove references to 'vendored plugins' except to note they used to exist but are deprecated. 'gemified plugin' is the only supported option now.

Added warning that require_tree includes files in unspecified order. So, that people may know the order of their scripts might be the reason that their scripts work in just fine in development but not in production. 

This will remove a lot of controversy.
As an example, see https://github.com/ernie/squeel/issues/67#issuecomment-5270896

Adds favicon to guides layout

This reverts commit 7b7cc134.

Reason: docrails isn't a rails fork. See 5444ed56

[ci skip]

section

Catch nil.to_sym errors in partial_renderer, and raise ArgumentError instead

So apps that accidentally add middlewares later aren't unwittingly dumping them in a black hole.

Closes #5911