Improve Contributing to Rails Guide

Logically group the content, so it makes more sense if someone tries to read from start of page to end of page. [ci skip]
Add documentation for Thread#freeze
Instead boat load of libraries, simply say StdLib
Use path where extensions are defined, not used

For consistency with all other documentation, point to where these core
extensions are defined, not where the example code comes from.

[ci skip]
Document LoadError core extensions

[ci skip]
Fix a few grammar issues. [ci skip]
Correct docs as singlton true is not returned here

[ci skip]
Fix spelling errors and grammar issues

Including replacing a few "smart quotes" with regular quotes (' and "),
for consistency.
[ci skip]
Remove wrapping div with inline styles for hidden form fields.

We are dropping HTML 4.01 and XHTML strict compliance since input
tags directly inside a form are valid HTML5, and the absense of
inline styles help in validating for Content Security Policy.
[ci skip] Use valid current config in example

`reaping_frequency` is used in Active Record `reap_frequency` is not
re-phrase how to run a test
Provide interface for accessing underlying IO object

In some cases users may need to work with/manipulate more of the
Tempfile api than provided by Upload. Allow users to get at the
underlying io via the common to_io method of IO/IO-like objects
[ci skip] PP does not generate pass by default

This was changed in https://github.com/rails/rails/pull/13415/files
[ci skip] document `statement_limit` 

Per discussion at https://github.com/rails/rails/issues/14645#issuecomment-40499409
CSRF protection should rescue exception not extend

I think the changes to the default behaviour mean that rails will throw an exception when an invalid authenticity token is found.  The previous proposed code of calling super then sign_out meant that sign_out was never reached - the exception handler never returned.

I think the best approach now is to catch the exception, although I'm not 100% certain on that.
Bring SQLite3Adpter init API closer to others
Merge pull request #14788 from arthurnn/conn_adpt_api

Bring SQLite3Adpter init API closer to others
Merge pull request #14755 from timlinquist/to_io_http_upload

Use common to_io so users can access the underlying IO object
Implement to_io as an alias
Change the method description