feature #5117 Complete review of the "Customize Error Pages" cookbook article (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

Complete review of the "Customize Error Pages" cookbook article

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

In my opinion the current article looks "old" compared to other parts of the documentation. I've reworded it completely to make it more actionable and to explain more clearly the three alternatives provided to developers.

Commits
-------

faad683 Address most of the comments made by Wouter
3842b61 Changes suggested by reviewers
3073783 Rewordings
722b9d0 Fixed the location of the cookbook article images
e304a9d Fixed a restructuredtext syntax issue
5ea54bf Fixed a reStructuredText formatting issue
b54a050 Complete review of the "Customize Error Pages" cookbook article

Author: weaverryan