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]
[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
[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
Fix spelling and proper nouns

[ci skip]
Merge branch 'master' of github.com:rails/docrails